PURPOSE: To enhance the efficiency of a supercharger by providing vacuum compensating pipes which are opened to gas carrying sections between adjacent leaves of a Roots rotor in a three or four leaf rotor type Roots blower and which are branch off from an intake-air passage, upstream of a throttle valve in a manifold connected to the suction port of the Roots blower.

CONSTITUTION: A second manifold 22 in which a Roots blower and a gate 19 are disposed is laid along a first man fold 11 which is communicated with cylinders in an engine E and in which an EFI instrument storage box 18 and a gate 23. The Roots blower, is of such a type that a three-leaf type Roots rotor 4 is disposed rotatably in a casing 1. In such an arrangement, vacuum compensating pipes 26 which branch off from an intake-air passage, upstream of a throttle valve 17, are provided to be communicated respectively with openings 8-1, 8-2 in a housing 5 defining the side surface of gas carrying section 6 between adjacent leaves of the rotors. With this arrangement, the degree of vacuum between the throttle valve 17 and the casing suction port 2 may be compensated, thereby it is possible to maintain the efficiency of supercharge at a high degree.