To provide an electric power steering device in which a control unit is mounted without forming a clearance to a gearbox mounting face of a reduction gearbox.

In a cover plate of a cover of a control unit 19, which opposes to the unit placing face of the reduction gearbox 4, a notch to make a plurality of external connection terminals pass through is formed. The unit placing face and the notch disposed in the cover plate are sealed by mounting a sealing member 33, and thus, the skirt portion of the sealing member 33 is brought into close contact with the cover plate while sealing the clearance.