To increase the structural rigidity of outer walls of a cylinder block wherein the coolant jacket thereof is adapted to receive coolant in a liquid state and discharge same in a gaseous state, a measure is employed wherein walls of the cylinders are integrally connected at their peripheral portions and a plurality of ribs are disposed in the coolant jacket with their laterally opposed sides integrally connected with the cylinder walls and the outer walls. Each rib extends essentially along the length of the cylinders from the bottom portion of the coolant jacket thereby to form a plurality of cells in the coolant jacket.