A motor-driven compressor includes a housing assembly, a compression mechanism, an electric motor, a discharge port, an outlet, a discharge passage, discharge valve and a valve device. The compression mechanism is accommodated in the housing assembly. The electric motor drives the compression mechanism. The discharge chamber is formed in the housing assembly. The discharge port is formed in the housing assembly for communication between the discharge chamber and the compression mechanism. The outlet is formed in the housing assembly for communication with an external circuit. The discharge passage is formed in the housing assembly for communication between the discharge chamber and the outlet. The discharge valve is disposed in the discharge chamber for opening and closing the discharge port. The valve device is configured to adjust an opening of the discharge passage.