The masking member comprises a matching surface mask portion to mask the matching surface with the cylinder block, port hole mask portions to mask openings of the intake ports and the exhaust ports, and a between openings mask portion to mask a region which is sandwiched between the openings of two adjacent port holes. The matching surface mask portion is linked directly to the port hole mask portions without any steps, and the between openings mask portion is linked directly to both of the port hole mask portions without a step.