A top plate is provided in a top portion of a cylinder block of a combustion engine of a type, provided with a cooling fan, so as to lie perpendicular to a longitudinal axis of an engine cylinder. An intake side gasket interposed between the cylinder block and an intake unit is extended upwardly to contact the top plate to thereby seal a gap between the top plate and the intake side gasket. An exhaust side gasket interposed between the cylinder block and an exhaust unit is extended upwardly to contact the top plate to thereby seal a gap between the top plate and the exhaust side gasket. A cooling passage for passing therethrough a cooling air from the cooling fan is formed by the cylinder block, the top plate, the intake side gasket and the exhaust side gasket.
