PURPOSE: To maintain high volume efficiency throughout a wide rotational range by providing a volume type of supercharger driven by an engine on the way of an intake path and an adjusting means for varying the equivalent pipe length of the intake path on the basis of engine load at the upstream side of the supercharger.

CONSTITUTION: An internal combustion engine with a supercharger for a motor cycle is provided on the upper portion of a crank case 7 with a supercharger 3 which is constituted to insert a sub-piston 19 reciprocated through a connecting rod 9 by a crankshaft 8 into a sub-cylinder 18. Then, an intake path 2a at the upstream side of the supercharger 3 is provided with an adjusting means 4 for varying the equivalent pipe length of said path 2a. This adjusting means 3 is constituted from a breeches pipe 27, a change-over valve 28 mounted on the branch portion thereof and an actuator 29 for driving said valve 28. One breech 27a of said pipe 27 is formed shorter than the other breech 27b and the change-over valve 28 is changed over to the position shown in the drawing during low rotation (low load) and the equivalent pipe length of the intake path 2a is set long.