diff --git a/.gitignore b/.gitignore
index a5567a0271..5517f80fad 100644
--- a/.gitignore
+++ b/.gitignore
@@ -116,6 +116,7 @@ lcov*.info
 /run.gdb
 /sizes.c
 /test.rb
+/test-coverage.dat
 /tmp
 /transdb.h
 /uncommon.mk
diff --git a/test/runner.rb b/test/runner.rb
index 1fe32fcfc8..02cc4610df 100644
--- a/test/runner.rb
+++ b/test/runner.rb
@@ -26,14 +26,7 @@ require_relative 'lib/zombie_hunter'
 require_relative 'lib/iseq_loader_checker'
 
 if ENV['COVERAGE']
-  %w[doclie simplecov-html simplecov].each do |f|
-    $LOAD_PATH.unshift "#{src_testdir}/../coverage/#{f}/lib"
-  end
-
-  require 'simplecov'
-  SimpleCov.start do
-    add_filter "/test/"
-  end
+  require "tool/test-coverage.rb"
 end
 
 begin
diff --git a/tool/test-coverage.rb b/tool/test-coverage.rb
new file mode 100644
index 0000000000..0c002f4d1a
--- /dev/null
+++ b/tool/test-coverage.rb
@@ -0,0 +1,78 @@
+require "coverage"
+
+Coverage.start
+
+TEST_COVERAGE_DATA_FILE = "test-coverage.dat"
+
+def merge_coverage_data(res1, res2)
+  res1.each do |path, cov1|
+    cov2 = res2[path]
+    if cov2
+      cov1.each_with_index do |count1, i|
+        next unless count1
+        if cov2[i]
+          cov2[i] += count1
+        else
+          cov2[i] = count1
+        end
+      end
+    else
+      res2[path] = cov1
+    end
+  end
+  res2
+end
+
+def save_coverage_data(res1)
+  File.open(TEST_COVERAGE_DATA_FILE, File::RDWR | File::CREAT | File::BINARY) do |f|
+    f.flock(File::LOCK_EX)
+    s = f.read
+    res2 = s.size > 0 ? Marshal.load(s) : {}
+    res1 = merge_coverage_data(res1, res2)
+    f.rewind
+    f << Marshal.dump(res2)
+    f.flush
+    f.truncate(f.pos)
+  end
+end
+
+def invoke_simplecov_formatter
+  %w[doclie simplecov-html simplecov].each do |f|
+    $LOAD_PATH.unshift "#{__dir__}/../coverage/#{f}/lib"
+  end
+
+  require "simplecov"
+  res = Marshal.load(File.binread(TEST_COVERAGE_DATA_FILE))
+  simplecov_result = {}
+  base_dir = File.dirname(__dir__)
+
+  res.each do |path, cov|
+    next unless path.start_with?(base_dir)
+    next if path.start_with?(File.join(base_dir, "test"))
+    simplecov_result[path] = cov
+  end
+
+  res = SimpleCov::Result.new(simplecov_result)
+  res.command_name = "Ruby's `make test-all`"
+  SimpleCov::Formatter::HTMLFormatter.new.format(res)
+end
+
+pid = $$
+pwd = Dir.pwd
+
+at_exit do
+  exit_exc = $!
+
+  Dir.chdir(pwd) do
+    save_coverage_data(Coverage.result)
+    if pid == $$
+      begin
+        nil while Process.waitpid(-1)
+      rescue Errno::ECHILD
+        invoke_simplecov_formatter
+      end
+    end
+  end
+
+  raise exit_exc if exit_exc
+end