The crankcase of an internal combustion engine includes a blow-by gas passage that is formed inside the crankcase and discharges blow-by gas inside the crankcase to the top of the crankcase. An oil drain passage is also formed inside the crankcase such that it discharges oil collected from the top of the crankcase into the crankcase, whereas the blow-by gas passage and the oil drain passage are interconnected with each other inside the crankcase.