diff --git a/gc.c b/gc.c
index b0c9041948..b9c1305060 100644
--- a/gc.c
+++ b/gc.c
@@ -2403,16 +2403,6 @@ Init_heap(void)
     heap_add_pages(objspace, heap_eden, gc_params.heap_init_slots / HEAP_PAGE_OBJ_LIMIT);
     init_mark_stack(&objspace->mark_stack);
 
-#ifdef USE_SIGALTSTACK
-    {
-	/* altstack of another threads are allocated in another place */
-	rb_thread_t *th = GET_THREAD();
-	void *tmp = th->altstack;
-	th->altstack = malloc(rb_sigaltstack_size());
-	free(tmp); /* free previously allocated area */
-    }
-#endif
-
     objspace->profile.invoke_time = getrusage_time();
     finalizer_table = st_init_numtable();
 }
diff --git a/internal.h b/internal.h
index e72785be56..3515767b36 100644
--- a/internal.h
+++ b/internal.h
@@ -1704,7 +1704,6 @@ int rb_match_nth_defined(int nth, VALUE match);
 /* signal.c */
 extern int ruby_enable_coredump;
 int rb_get_next_signal(void);
-int rb_sigaltstack_size(void);
 
 /* st.c */
 extern void rb_hash_bulk_insert(long, const VALUE *, VALUE);
diff --git a/signal.c b/signal.c
index 694661457b..5803f8f0b9 100644
--- a/signal.c
+++ b/signal.c
@@ -554,7 +554,7 @@ typedef RETSIGTYPE ruby_sigaction_t(int);
 #endif
 
 #ifdef USE_SIGALTSTACK
-int
+static int
 rb_sigaltstack_size(void)
 {
     /* XXX: BSD_vfprintf() uses >1500KiB stack and x86-64 need >5KiB stack. */
@@ -577,19 +577,18 @@ rb_sigaltstack_size(void)
 }
 
 /* alternate stack for SIGSEGV */
-void
-rb_register_sigaltstack(rb_thread_t *th)
+void *
+rb_register_sigaltstack(void)
 {
     stack_t newSS, oldSS;
 
-    if (!th->altstack)
-	rb_bug("rb_register_sigaltstack: th->altstack not initialized\n");
-
-    newSS.ss_sp = th->altstack;
     newSS.ss_size = rb_sigaltstack_size();
+    newSS.ss_sp = xmalloc(newSS.ss_size);
     newSS.ss_flags = 0;
 
     sigaltstack(&newSS, &oldSS); /* ignore error. */
+
+    return newSS.ss_sp;
 }
 #endif /* USE_SIGALTSTACK */
 
@@ -1532,9 +1531,7 @@ Init_signal(void)
 	install_sighandler(SIGILL, (sighandler_t)sigill);
 #endif
 #ifdef SIGSEGV
-# ifdef USE_SIGALTSTACK
-	rb_register_sigaltstack(GET_THREAD());
-# endif
+	RB_ALTSTACK_INIT(GET_VM()->main_altstack);
 	install_sighandler(SIGSEGV, (sighandler_t)sigsegv);
 #endif
     }
diff --git a/thread.c b/thread.c
index f0ace8326c..5a4726d1ee 100644
--- a/thread.c
+++ b/thread.c
@@ -652,11 +652,6 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start, VALUE *register_stack_s
     rb_thread_list_t *join_list;
     rb_thread_t *main_th;
     VALUE errinfo = Qnil;
-# ifdef USE_SIGALTSTACK
-    void rb_register_sigaltstack(rb_thread_t *th);
-
-    rb_register_sigaltstack(th);
-# endif
 
     if (th == th->vm->main_thread)
 	rb_bug("thread_start_func_2 must not be used for main thread");
diff --git a/thread_pthread.c b/thread_pthread.c
index 951885ffa0..43766b26ba 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -840,6 +840,7 @@ static void *
 thread_start_func_1(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
+    RB_ALTSTACK_INIT(void *altstack);
 #if USE_THREAD_CACHE
   thread_start:
 #endif
@@ -868,6 +869,7 @@ thread_start_func_1(void *th_ptr)
 	}
     }
 #endif
+    RB_ALTSTACK_FREE(altstack);
     return 0;
 }
 
diff --git a/vm.c b/vm.c
index 1a57f2c6d1..dd67062ee5 100644
--- a/vm.c
+++ b/vm.c
@@ -2216,6 +2216,7 @@ ruby_vm_destruct(rb_vm_t *vm)
 	    vm->frozen_strings = 0;
 	}
 	rb_vm_gvl_destroy(vm);
+	RB_ALTSTACK_FREE(vm->main_altstack);
 	if (objspace) {
 	    rb_objspace_free(objspace);
 	}
@@ -2473,9 +2474,6 @@ thread_free(void *ptr)
 	RUBY_GC_INFO("main thread\n");
     }
     else {
-#ifdef USE_SIGALTSTACK
-	free(th->altstack);
-#endif
 	ruby_xfree(ptr);
     }
 
@@ -2535,11 +2533,6 @@ th_init(rb_thread_t *th, VALUE self)
     th->self = self;
     rb_threadptr_root_fiber_setup(th);
 
-    /* allocate thread stack */
-#ifdef USE_SIGALTSTACK
-    /* altstack of main thread is reallocated in another place */
-    th->altstack = malloc(rb_sigaltstack_size());
-#endif
     {
 	/* vm_stack_size is word number.
 	 * th->vm->default_params.thread_vm_stack_size is byte size. */
diff --git a/vm_core.h b/vm_core.h
index 5fa11487f7..a5ba1900c1 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -101,7 +101,13 @@
 #endif
 
 #if defined(SIGSEGV) && defined(HAVE_SIGALTSTACK) && defined(SA_SIGINFO) && !defined(__NetBSD__)
-#define USE_SIGALTSTACK
+#  define USE_SIGALTSTACK
+void *rb_register_sigaltstack(void);
+#  define RB_ALTSTACK_INIT(var) var = rb_register_sigaltstack()
+#  define RB_ALTSTACK_FREE(var) xfree(var)
+#else /* noop */
+#  define RB_ALTSTACK_INIT(var)
+#  define RB_ALTSTACK_FREE(var)
 #endif
 
 /*****************/
@@ -538,6 +544,9 @@ typedef struct rb_vm_struct {
 
     struct rb_thread_struct *main_thread;
     struct rb_thread_struct *running_thread;
+#ifdef USE_SIGALTSTACK
+    void *main_altstack;
+#endif
 
     rb_serial_t fork_gen;
     struct list_head waiting_fds; /* <=> struct waiting_fd */
@@ -883,9 +892,6 @@ typedef struct rb_thread_struct {
     /* misc */
     unsigned int abort_on_exception: 1;
     unsigned int report_on_exception: 1;
-#ifdef USE_SIGALTSTACK
-    void *altstack;
-#endif
     uint32_t running_time_us; /* 12500..800000 */
     VALUE name;
 } rb_thread_t;