A motor-driven compressor (10, 70) that is used for a vehicle and horizontally disposed, comprising: an electric motor including a rotary shaft; a compression mechanism driven by the electric motor so as to compress refrigerant gas; a metal housing accommodating the electric motor and the compression mechanism; a suction passage communicable with interior of the housing, the suction passage through which refrigerant gas flows; and a discharge passage communicable with the interior of the housing, the discharge passage through which refrigerant gas discharged from the compression mechanism flows: a discharge chamber connected to the discharge passage, wherein refrigerant gas compressed in a compression chamber is discharged to the discharge chamber, and flows through the discharge passage to the external refrigerant circuit; a discharge port provided in the compression mechanism that connects the compression chamber with the discharge chamber to discharge the compressed refrigerant to the discharge chamber; a discharge valve provided in the compression mechanism to open and close the discharge port; and a plurality of conductive parts (45, 46, 48) that is disposed in a vicinity of the electric motor and above the rotary shaft; characterized in that the motor-driven compressor further includes: a passage configured to communicate an oil separator provided upstream to the discharge passage with the compression mechanism bypassing the discharge chamber; a check valve (51, 52) that is provided in at least one of the suction passage and the discharge passage, opened while the compressor (10, 70) is in operation and closed while the compressor (10, 70) is at a stop. an electric motor including a rotary shaft; a compression mechanism driven by the electric motor so as to compress refrigerant gas; a metal housing accommodating the electric motor and the compression mechanism; a suction passage communicable with interior of the housing, the suction passage through which refrigerant gas flows; and a discharge passage communicable with the interior of the housing, the discharge passage through which refrigerant gas discharged from the compression mechanism flows: a discharge chamber connected to the discharge passage, wherein refrigerant gas compressed in a compression chamber is discharged to the discharge chamber, and flows through the discharge passage to the external refrigerant circuit; a discharge port provided in the compression mechanism that connects the compression chamber with the discharge chamber to discharge the compressed refrigerant to the discharge chamber; a discharge valve provided in the compression mechanism to open and close the discharge port; and a plurality of conductive parts (45, 46, 48) that is disposed in a vicinity of the electric motor and above the rotary shaft; characterized in that the motor-driven compressor further includes: a passage configured to communicate an oil separator provided upstream to the discharge passage with the compression mechanism bypassing the discharge chamber; a check valve (51, 52) that is provided in at least one of the suction passage and the discharge passage, opened while the compressor (10, 70) is in operation and closed while the compressor (10, 70) is at a stop. a discharge chamber connected to the discharge passage, wherein refrigerant gas compressed in a compression chamber is discharged to the discharge chamber, and flows through the discharge passage to the external refrigerant circuit; a discharge port provided in the compression mechanism that connects the compression chamber with the discharge chamber to discharge the compressed refrigerant to the discharge chamber; a discharge valve provided in the compression mechanism to open and close the discharge port; and a plurality of conductive parts (45, 46, 48) that is disposed in a vicinity of the electric motor and above the rotary shaft; characterized in that the motor-driven compressor further includes: a passage configured to communicate an oil separator provided upstream to the discharge passage with the compression mechanism bypassing the discharge chamber; a check valve (51, 52) that is provided in at least one of the suction passage and the discharge passage, opened while the compressor (10, 70) is in operation and closed while the compressor (10, 70) is at a stop. a passage configured to communicate an oil separator provided upstream to the discharge passage with the compression mechanism bypassing the discharge chamber; a check valve (51, 52) that is provided in at least one of the suction passage and the discharge passage, opened while the compressor (10, 70) is in operation and closed while the compressor (10, 70) is at a stop.