An internal combustion engine, comprising: a first cylinder, a second cylinder, a third cylinder, and a fourth cylinder which are cylinders arranged in order in a first direction along an axis of a crankshaft; a first crank chamber connected to the first cylinder, a second crank chamber connected to the second cylinder, a third crank chamber connected to the third cylinder, and a fourth crank chamber connected to the fourth cylinder; an oil chamber that is connected to the first crank chamber, the second crank chamber, the third crank chamber, and the fourth crank chamber on a side opposite to the cylinders, the oil chamber storing oil; a first piston configured to reciprocate in the first cylinder, a second piston configured to reciprocate in an opposite phase to the first piston in the second cylinder, a third piston configured to reciprocate in an opposite phase to the first piston in the third cylinder, and a fourth piston configured to reciprocate in a same phase as the first piston in the fourth cylinder; a first wall that defines the first crank chamber from a side in a second direction opposite to the first direction, a second wall that separates the first crank chamber and the second crank chamber from each other, a third wall that separates the second crank chamber and the third crank chamber from each other, a fourth wall that separates the third crank chamber and the fourth crank chamber from each other, and a fifth wall that defines the fourth crank chamber from a side in the first direction; a first blow-by gas passage defined in the second wall and connecting the oil chamber and an oil separator, a second blow-by gas passage defined in the third wall and connecting the oil chamber and the oil separator, and a third blow-by gas passage defined in the fourth wall and connecting the oil chamber and the oil separator; and a first connecting path defined in the second wall and connecting the first crank chamber and the second crank chamber to each other, and a second connecting path defined in the fourth wall and connecting the third crank chamber and the fourth crank chamber to each other, wherein the third wall has no passage connecting the second crank chamber and the third crank chamber to each other. a first cylinder, a second cylinder, a third cylinder, and a fourth cylinder which are cylinders arranged in order in a first direction along an axis of a crankshaft; a first crank chamber connected to the first cylinder, a second crank chamber connected to the second cylinder, a third crank chamber connected to the third cylinder, and a fourth crank chamber connected to the fourth cylinder; an oil chamber that is connected to the first crank chamber, the second crank chamber, the third crank chamber, and the fourth crank chamber on a side opposite to the cylinders, the oil chamber storing oil; a first piston configured to reciprocate in the first cylinder, a second piston configured to reciprocate in an opposite phase to the first piston in the second cylinder, a third piston configured to reciprocate in an opposite phase to the first piston in the third cylinder, and a fourth piston configured to reciprocate in a same phase as the first piston in the fourth cylinder; a first wall that defines the first crank chamber from a side in a second direction opposite to the first direction, a second wall that separates the first crank chamber and the second crank chamber from each other, a third wall that separates the second crank chamber and the third crank chamber from each other, a fourth wall that separates the third crank chamber and the fourth crank chamber from each other, and a fifth wall that defines the fourth crank chamber from a side in the first direction; a first blow-by gas passage defined in the second wall and connecting the oil chamber and an oil separator, a second blow-by gas passage defined in the third wall and connecting the oil chamber and the oil separator, and a third blow-by gas passage defined in the fourth wall and connecting the oil chamber and the oil separator; and a first connecting path defined in the second wall and connecting the first crank chamber and the second crank chamber to each other, and a second connecting path defined in the fourth wall and connecting the third crank chamber and the fourth crank chamber to each other, wherein the third wall has no passage connecting the second crank chamber and the third crank chamber to each other.