To provide a balancer for a reciprocating piston engine improved so as to be unhinderable in engine compactness.

This balancer has two balancer shafts 10L and 10R: gear-joined with each other, chain/sprocket mechanisms 9, 11, and 12: for interlockingly connecting between at least one side balancer shaft 10L and a crank shaft 1, a chain tensioner 14: for automatically adjusting the tightening of the chain 12, and a pump receiving part: integrally provided with a balancer shaft holder for supporting the balancer shaft in order to receive the rotor of a lubricating oil pump. In this case, the rotor of the lubricating oil pump is to be directly connected to another side balancer shaft 10R, and the chain tensioner is to be arranged in the shaft end side of another side balancer shaft. Consequently the chain tensioner can be arranged within a range sharply unprotruded from the triangular profile constituted of two balancer shafts and the crank shaft.