PURPOSE: To make it possible to easily fill up lubricating oil by inserting, into the both sides of a joint connecting a compensator to a crank chamber, check valves for closing a communicating part only when the compensator is removed.

CONSTITUTION: A collar 13a on the tip of a flange 13 of a compensator C is tightened by a nut 18 while putting a joint 1b and an O ring 17 provided on a crank case 1 inside the collar. In a state the compensator C has been mounted, the projections 19a of check valves 19 provided on the mounting parts of the compensator and the crank chamber press to each other to produce an oil passage between a valve seat 13b and the check valve, so that oil tightness between a compensator chamber 14a and the crank chamber 1a is kept by the check valve 19 when the compensator is removed, preventing the oil from leaking outside.