To provide a speed reducer, reduced in size by a comparatively simple mechanism.

This speed reducer includes: a hollow cylindrical shaft part 6 rotatably supported in slit cases 2, 3; a worm gear mechanism 5 for reducing the rotation of a motor and transmitting the same to the cylindrical shaft part 6; an eccentric shaft part 20 formed with eccentricity to the axis X of rotation of the cylindrical shaft part 6; a differential gear mechanism 7 having a differential external gear 22 fixed to the cases 2, 3 and a differential internal gear 21 externally journaled to the eccentric shaft part 20 and circumscribing the differential external gear 22 to mesh therewith; a differential phase takeout part 8 for taking out the phase lag of the differential internal gear 21 corresponding to a difference in number of teeth of the differential gear mechanism 7; an output shaft part 9 passed through the cylindrical shaft part 6 with both ends extended to the outside of the cases 2, 3 and rotatably supported on the cases 2, 3; and a transmission part 10 for transmitting the phase lag to the output shaft part 9.