To provide an internal combustion engine, in which increase in size viewed from an end side of a crankshaft is suppressed.

The engine (internal combustion engine) 15 comprises: a crankcase 22; a crankshaft 31 housed in the crankcase 22; a first crank gear 32 and a second crank gear 33 disposed in the crankcase 22 and rotated around the crankshaft 31; a balancer gear 35 meshed with the first crank gear 32, and rotated along with the rotation of the first crank gear 32; and a water pump part 54 having a water pump gear 56. The water pump gear 56 is meshed with the second crank gear 33 and rotated along with the rotation of the second crank gear 33. The balancer gear 35 and the water pump gear 56 are arranged to be overlapped with each other when viewed from the end side of the crankshaft 31.