PURPOSE: To reduce the size of a device, by setting bearings for supporting a crankshaft on both sides with a relatively long distance which can also serve as a valve body of a rotary selector valve.

CONSTITUTION: A relatively long bearing boss 21 is formed on an end of a casing 20, being projected on both sides of the casing 20. A pair of radial bearings 22, by which a crankshaft 23 is supported, are set on both ends of the boss 21. Hydraulic cylinders 25 are radially arranged and connected, through an overhung crank mechanism 24, with the crankshaft 23 in the casing 20. The crankshaft 23 also serves as a rotary valve body 34 of a rotary selector valve 33 between the pair of bearings 22 which are opposed with a bit longer distance to each other in the axial direction. This structure can reduce the required number of parts and minimize the size.