PROBLEM TO BE SOLVED: To increase engine output by increasing operating frequencies of an electric motor to shorten a rest time through the enhancement of the cooling function of an inverter and electric motor in an electric supercharger, thereby improving a transient response. SOLUTION: An electric supercharger consists of: a charging device 05; and an inverter 3 for converting power of the charging device 05 to supply it to an electric motor 2, and supercharges supply air (a) of an internal combustion engine 06 by driving a compressor 1 with the electric motor 2. An intake air flow passage 10 is formed of: a serial flow passage 12 reaching the compressor 1 after passing through the electric motor 2 and a heat generation part in the inverter 3; a first bypass flow passage 14 reaching the compressor 1 after bypassing the electric motor 2 and inverter 3; and a second bypass flow passage 15 reaching the internal combustion engine after bypassing the electric motor 2, inverter 3 and compressor 1. The supply air a is switchably supplied so as to pass through either of the serial flow passage 12, first bypass flow passage 14, and second bypass flow passage 15. COPYRIGHT: (C)2008,JPO&INPIT