To provide an electric steering assisting device capable of preventing generation of a hitting sound and the deterioration in an operation sound, even when there is alignment failure between both shafts of a rotary shaft and a worm shaft of an electric motor.

Axial dislocation between the rotary shaft 21 and the worm shaft 13 of the electric motor 11 caused by the alignment failure is absorbed by a joint unit provided with an elastic member. A point of application of a load caused by the alignment failure is brought to the midpoint 30 of the center of an area in the axial direction of a motor side bearing 35, and is thereby prevented from acting on the rear side of the electric motor 11, that is, on a rear side rotary shaft 34 and a rear bearing 36.