To prevent dew condensation in a crankcase in a lower part of the crankcase, wherein an oil pan is an integral part of an internal combustion engine, to prevent lubricating oil from being diluted. A crankcase cover covers at least an oil pan out of a crankcase and is provided to the bottom of the crankcase. A heat insulating material is provided between the crankcase and the crankcase cover. A cooling fluid passage for providing a circulation to cool a cylinder or a cylinder head is provided between the crankcase and the crankcase cover. A passage at least one end of which reaches a cylinder head from a crankcase via a cylinder block forms at least a part of a blowby gas passage, is directly provided to the crankcase, the cylinder block and the cylinder head. A flow control valve is arranged in the cylinder block or the cylinder head.