A cylinder block of a multicylinder engine, having a cylinder main body in which multiple cylinder bores are provided in parallel in an axial direction of a crankshaft, and a crankcase having plural journal walls. The crankcase is integrally provided with the cylinder main body, in which a communication hole extending in parallel to the axial line of the crankshaft is provided in the cylinder main body and the crankcase while at least a part of which is opened in the inner peripheral surface of the cylinder bore. This permits the cylinder block to be downsized, increases freedom of positional setting of the communication hole, and reduces ventilation resistance of air flow through the communication hole. A cut-processed member is expanded further outward from a piston slide surface and is formed along a radial direction of cylinder bores, in inner surfaces of the cylinder bores in at least portions closer to a piston at an open edge of a communication hole.