Abstract: A water jacket spacer has a spacer body molded in conformity to the shape of the water jacket and a regulating member whose one side is fixed to the spacer body. The regulating member is composed of a first member which is not water-swellable and is elastically deformable and a second member made of water-swellable elastomer which is integrally fixed to the first member. The regulating member lies in a state not-contacting an opposed wall of the water jacket when inserted into the water jacket together with the spacer body, whereas it blocks a part of cooling water flowing through the water jacket by deforming such that a side of the regulating member not fixed to the spacer body faces toward the opposed wall of the water jacket when the second member swells by absorbing cooling water flown into the water jacket and thereby the first member is elastically deformed.