PURPOSE: To obtain high output at all engine speed by forming an auxiliary chamber which is communicated with a crank chamber of an engine between the crank chamber and boss parts attached to a vehicle body frame, and varying the communication condition via an opening and closing valve in response to the engine speed.

CONSTITUTION: In an engine 1, respective attached boss parts 12, 13 formed monolithically with a case 11 are fixed to a vehicle frame via bolt holes 14 - 16. In this case, a space between a crank chamber 3 and the attached boss part 12 is used to form and an auxiliary chamber 18 communicated by a communication hole 17. A solenoid valve 19 which opens and closes the communication hole 17 is provided. At a low middle engine speed range, a high output is obtained by closing the communication hole 17 by means of the solenoid valve 19 and improving a pressure ratio in the crank chamber 3. At a high rotation range, on the other hand, the high output is obtained opening communication hole 17 by means of the solenoid valve 19, and by lowering the pressure ratio in the crank chamber 3 and the auxiliary chamber 18. The high output is thus obtained at all the speed range of the engine 1.