A harmonic drive according to the present invention comprises a plurality of internal gears; a band type of flexible gear provided with teeth engaged with each of the internal gears on its outer circumferential surface, the flexible gear having a pitch circumference length shorter than that of said internal gears; and a wave generating part disposed on an inner side of said flexible gear, the wave generating part generating a plurality of harmonic motions by deforming said flexible gear as the wave generating part rotates. Further, said flexible gear is formed integrally to engage with said plurality of internal gears so that the plurality of harmonic motions are overlapped, and one or more of said plurality of internal gears and said flexible gear are formed of profile shifted gears.