An internal combustion engine includes a bottomed box-shaped oil pan in which oil is stored. In addition, the internal combustion engine includes an oil pump configured to suck the oil in the oil pan and pressure-feed the oil to each part of the internal combustion engine. A suction pipe extends toward a bottom surface of the oil pan from the oil pump. A strainer is attached to a tip end of the suction pipe. A projection portion protrudes from a bottom surface which is one of inner wall surfaces defining an oil storing space inside the oil pan. The resonance frequency of the projection portion falls in the range of frequency of vibration that is generated at an engine rotational speed at which the internal combustion engine continues to operate independently.