To miniaturize an engine itself by miniaturizing a balancer shaft as small as possible.

In the engine wherein the balancer shaft 80 is mounted in parallel with a crank shaft 5, and rotated from the crank shaft 5 through gears 82, 83, a fitting part 90 for fitting the gear 83 is mounted on an end part of the balancer shaft 80, a flange part 90b is formed on a machine body central side of the fitting part, a stepped part 83b is formed on the machine body central-side end part of a shaft hole 83a formed on the gear 83, a recess 80e is formed on the balancer shaft at a side opposite to a balance weight 80c mounted on the balancer shaft, and a bottom part of the recess 80e is located at a balance weight side with respect to a shaft center of the balancer shaft.