diff --git a/bignum.c b/bignum.c
index 8f351ee208..75651f9524 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1645,6 +1645,12 @@ rb_big_sq_fast(VALUE x)
     return z;
 }
 
+static inline size_t
+max_size(size_t a, size_t b)
+{
+    return (a > b ? a : b);
+}
+
 /* balancing multiplication by slicing larger argument */
 static void
 bary_mul_balance_with_mulfunc(BDIGIT *const zds, const size_t zn,
@@ -1662,8 +1668,14 @@ bary_mul_balance_with_mulfunc(BDIGIT *const zds, const size_t zn,
     BDIGITS_ZERO(zds, xn);
 
     if (wn < xn) {
-        const size_t r = (yn % xn) ? (yn % xn) : xn;
-        if ((2 * xn + yn + r) > zn) {
+        /* The condition when a new buffer is needed:
+         * 1. (2(xn+r) > zn-(yn-r)) => (2xn+r > zn-yn), at the last
+         *    iteration (or r == 0)
+         * 2. (2(xn+xn) > zn-(yn-r-xn)) => (3xn-r > zn-yn), at the
+         *    previous iteration.
+         */
+        const size_t r = yn % xn;
+        if (2*xn + yn + max_size(xn-r, r) > zn) {
             wn = xn;
             wds = ALLOCV_N(BDIGIT, work, wn);
         }
diff --git a/test/ruby/test_bignum.rb b/test/ruby/test_bignum.rb
index 53a2b20cc8..065a944853 100644
--- a/test/ruby/test_bignum.rb
+++ b/test/ruby/test_bignum.rb
@@ -203,6 +203,15 @@ class TestBignum < Test::Unit::TestCase
     assert_equal(00_02, '00_02'.to_i)
   end
 
+  def test_very_big_str_to_inum
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      digits = [["3", 700], ["0", 2700], ["1", 1], ["0", 26599]]
+      num = digits.inject("") {|s,(c,n)|s << c*n}.to_i
+      assert_equal digits.sum {|c,n|n}, num.to_s.size
+    end;
+  end
+
   def test_to_s2
     assert_raise(ArgumentError) { T31P.to_s(37) }
     assert_equal("9" * 32768, (10**32768-1).to_s)