The position of a passage separating member in the axial direction of the cylinder bores is determined by causing a spacer to contact a bottom surface of a water jacket. When the separating member is inserted in the water jacket, the width of the separating member is reduced due to elastic deformation, so that the separating member can be arranged in the water jacket. After being arranged, the separating member tightly contacts the inner surface of the water jacket due to elastic restoration force. The tight contact prevents the separating member from moving upward in the water jacket. As a result, coolant is prevented from moving between the upper portion and the lower portion with respect to the separating member. The advantages of separate cooling of the coolant in the upper and lower portions with respect to the separating member are obtained. This reliably reduces the temperature difference along the axial direction of the cylinder bore forming body.