An electric power steering device comprising: a steering column in which a steering shaft through which steering torque is transmitted is installed therein; an electric motor for transmitting a steering assist force via a worm speed reduction mechanism in the speed reduction gear box to the steering shaft; a control unit for driving the electric motor; and a motor mounting part provided in the speed reduction gear box, for mounting the electric motor to which a motor shaft is joined to a worm meshed with the worm wheel of the speed reduction mechanism, wherein the electric motor includes a motor case (30; 35) in which a motor rotation part of which one end is opened and is formed in a cylindrical shape is built in, and a substantially disc-shaped flange member linked so that the one end of the motor opening closes, the flange member is provided with two mounting flanges fixed to the motor mounting part by outwardly protruding therefrom at an irregularly pitched position in a circumferential direction of the flange member, and at least one first abutting flange (31c; 37a; 37b) abutting to the motor mounting part by outwardly protruding between the two mounting flanges, a motor side external connecting part is protrudingly provided to connect to a unit side connecting part of the control unit at an outer circumference of the flange member, the first abutting flange (31c; 37a; 37b) and the motor side external connecting part are arranged symmetrically to a first phantom line orthogonal to a shaft center of the motor shaft, the two mounting flanges are arranged symmetrically to a second phantom line orthogonal on the same plane to the first phantom line, the first abutting flange (31c; 37a; 37b) is provided in front of a vehicle with respect to the first phantom line, the first abutting flange (31c, 37a; 37b) and the motor side external connecting part are arranged substantially symmetrically to the first phantom line, and two abutting flanges as the first abutting flange (31c; 37a; 37b) are provided apart from each other in a circumferential direction at a substantially line symmetry position with the motor side external connecting part, characterized in that a minute projection is provided at one abutting surface out of a first abutting surface of the first abutting flange (31c; 37a; 37b) and a second abutting surface of motor mounting part, the second abutting surface abutting to the first abutting surface, the minute projection locally abutting to the other abutting surface out of the first abutting surface and the second abutting surface. a steering column in which a steering shaft through which steering torque is transmitted is installed therein; an electric motor for transmitting a steering assist force via a worm speed reduction mechanism in the speed reduction gear box to the steering shaft; a control unit for driving the electric motor; and a motor mounting part provided in the speed reduction gear box, for mounting the electric motor to which a motor shaft is joined to a worm meshed with the worm wheel of the speed reduction mechanism, wherein the electric motor includes a motor case (30; 35) in which a motor rotation part of which one end is opened and is formed in a cylindrical shape is built in, and a substantially disc-shaped flange member linked so that the one end of the motor opening closes, the flange member is provided with two mounting flanges fixed to the motor mounting part by outwardly protruding therefrom at an irregularly pitched position in a circumferential direction of the flange member, and at least one first abutting flange (31c; 37a; 37b) abutting to the motor mounting part by outwardly protruding between the two mounting flanges, a motor side external connecting part is protrudingly provided to connect to a unit side connecting part of the control unit at an outer circumference of the flange member, the first abutting flange (31c; 37a; 37b) and the motor side external connecting part are arranged symmetrically to a first phantom line orthogonal to a shaft center of the motor shaft, the two mounting flanges are arranged symmetrically to a second phantom line orthogonal on the same plane to the first phantom line, the first abutting flange (31c; 37a; 37b) is provided in front of a vehicle with respect to the first phantom line, the first abutting flange (31c, 37a; 37b) and the motor side external connecting part are arranged substantially symmetrically to the first phantom line, and two abutting flanges as the first abutting flange (31c; 37a; 37b) are provided apart from each other in a circumferential direction at a substantially line symmetry position with the motor side external connecting part, characterized in that a minute projection is provided at one abutting surface out of a first abutting surface of the first abutting flange (31c; 37a; 37b) and a second abutting surface of motor mounting part, the second abutting surface abutting to the first abutting surface, the minute projection locally abutting to the other abutting surface out of the first abutting surface and the second abutting surface.