Abstract: The speed reduction device for the electrically powered foldable rearview mirror includes a two stage speed reduction worm mechanism connecting an electrically powered motor for folding the rearview mirror which is fixed on a frame of a mirror body and a clutch gear engageably and removably mounted on a shaft which stands on a base and on which the mirror body is rotatably mounted. The first stage worm of the speed reduction worm mechanism is a twin gear including a worm screw portion with a coaxial spur gear on an end thereof. The second stage worm is a twin gear including a worm screw portion with a coaxial worm wheel on an end thereof. The spur gear of the first stage worm is engaged with a pinion mounted on an output shaft of the motor but the axis of the worm screw portion of the first stage worm is parallel with the motor output shaft.