An internal combustion engine includes: a cylinder having a combustion chamber formed at an upper side thereof and accommodating a piston to be reciprocally movable; a crankcase provided below the cylinder and accommodates a crankshaft; and a blow-by gas passageway recirculating blow-by gas, which leaks into the crankcase from the combustion chamber, to the combustion chamber through an intake system, in which an inner surface portion of the crankcase is provided with a blow-by gas intake part, and the blow-by gas intake part includes a protruding portion protruding in a direction in which the crankshaft extends, and the intake port opened in the protruding direction is formed at a tip end portion of the protruding portion.