diff --git a/rubygem-pg.spec b/rubygem-pg.spec
index a0224a9..91f39a3 100644
--- a/rubygem-pg.spec
+++ b/rubygem-pg.spec
@@ -80,7 +80,9 @@ done
 pushd .%{gem_instdir}
 # Set --verbose to show detail log by $VERBOSE.
 # See https://github.com/ged/ruby-pg/blob/master/spec/helpers.rb $VERBOSE
-if ! ruby -S --verbose \
+# Assign a random port to consider a case of multi builds in parallel in a host.
+# https://github.com/ged/ruby-pg/pull/39
+if ! PGPORT="$((54321 + ${RANDOM} % 1000))" ruby -S --verbose \
   rspec -I$(dirs +1)%{gem_extdir_mri} -f d spec; then
   echo "==== [setup.log start ] ===="
   cat tmp_test_specs/setup.log