An internal combustion engine having an internal space including a crankcase chamber includes an oil tank, a first passage communicating a lower part of the internal space with a gas phase part of the oil tank, a first check valve provided in the first passage to permit a flow from the internal space to the oil tank, a second passage communicating the gas phase part with the internal space, a second check valve provided in the second passage to permit a flow from the second passage to the internal space, a third passage communicating a liquid phase part of the oil tank with the second passage, a third check valve provided in the third passage to permit a flow from the oil tank to the second passage, and a flow regulator valve provided in the third passage for regulating a flow of lubricating oil flowing through the third passage.