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 beings selectable from a wide range of materials 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 spaced from the steering pinion and meshing with the rack shaft, so as to assist steering. The worm speed reduction mechanism has a structure in which worms 24 and 25 connected to the respective output shafts of plural motors 35 and 36 simultaneously mesh with a worm wheel 23 connected integral with the auxiliary pinion 20.