A device includes a timing sprocket configured to receive a rotational force from a crankshaft and includes an annular internal-teeth constituting portion having an inner circumferential portion formed with internal teeth; an eccentric shaft portion provided on a motor output shaft of an electric motor and formed in a cylindrical shape such that an outer circumferential surface of the eccentric shaft portion is eccentric relative to a rotational center of the eccentric shaft portion; and a plurality of rollers provided between the internal teeth and the eccentric shaft portion and having total number smaller than total number of the internal teeth. A laser hardening is performed from a tooth top and both tooth surfaces of each internal tooth to attain a high degree of hardness. On the other hand, a tooth bottom surface of each internal tooth is not treated with the laser hardening.