A motor-driven supercharger is provided with a stator assembly inserted and attached to a bearing housing, and a seal plate closely attached to a compressor side of the stator assembly and comparting between a compressor housing and a motor stator (24). The stator assembly has the motor stator and an outer sleeve having an inner peripheral surface closely attached to an outer peripheral surface of the motor stator and supporting the motor stator in such a manner as to constrain a rotation of the motor stator, wherein the motor stator and the outer sleeve form an integral part so as to construct the stator assembly, and the outer sleeve constructs a liquid tight water cooling jacket between the outer sleeve and the bearing housing. The outer sleeve and the seal plate are fastened together in an axial direction between the bearing housing and the compressor housing.