To provide a speed reduction mechanism for improving the efficiency of transmitting assist torque during assist without disturbing the transmission of reverse input in a non-assist region, when applied to an electric power steering device.

A worm wheel 12 includes an annular outer ring 50 on the outer periphery of which teeth are formed, and an inner ring 51 connected to an output shaft. First and second power transmission members 53, 54 are embedded in an annular elastic member 52 which exists between the outer ring 50 and the inner ring 51 and to which both rings 50, 51 are directly connected. With predetermined or lower reverse input from a road surface during non-assist, the elastic member 52 is elastically deformed to cause the rotation of the inner ring 51 relative to the outer ring 50. Then, road surface information is transmitted to a driver. During assist, the power transmission members 53, 54 engage with the outer ring 50 and the inner ring 51, respectively, to transmit torque between both rings 50, 51.