To provide a motor-driven power steering device whose material strength can be set at a low value so that it can be made smaller in size and lighter in weight, with its materials being selectable from a wide range so that materials with excellent silentness or the like are usable.

In this motor-driven power steering device, a steering wheel is turned by the sliding movement of a rack shaft meshing with a steering pinion connected to the steering side, and a motor output controlled in accordance with steering torque on the steering side is transmitted via a worm speed reduction mechanism to an auxiliary pinion 20 spaced from the steering pinion and meshing with the rack shaft, so as to assist steering. The worm speed reduction mechanism 19 is so designed that a plurality of worms 24, 25, and 26 connected to the output shaft of the motor simultaneously mesh with a worm wheel 23 connected integral with the auxiliary piston 20.