An auxiliary machinery mounting structure of an engine which can reduce the entire length of the engine, enhance the rigidity of a cover seal surface to improve the sealing effect, and also enhance the mounting rigidity of an auxiliary machinery. The auxiliary machinery mounting structure includes a mount seat that is integrally formed on an end surface of a cylinder block extending vertically to a crank shaft of the engine to lie outward of and along a cover seal surface that is a surface engaging with a cover laid over the end surface, and the auxiliary machinery is mounted on the mount seat.