A water jacket spacer is inserted into a water jacket. The water jacket spacer comprises an expansion member, a plate-shaped holder to which the expansion member is fixed, and a leaf spring that abuts the inner wall of the water jacket. The leaf spring projects from a face on the holder on the side opposite of the expansion member. The holder curves to match the shape of the water jacket. The position of the water jacket spacer in the water jacket is maintained by the biasing force of the leaf spring.