A harmonic speed reducer is provided. The harmonic speed reducer includes a wave generator, a flexible gear, a first rigid gear, and a second rigid gear. The wave generator can be driven to rotate relative to a central axis. The flexible gear has a plurality of first outer gear structures, a division groove, and a plurality of second outer gear structures. The first rigid gear has a plurality of first inner gear structures configured to be engaged with the first outer gear structures. The second rigid gear has a plurality of second inner gear structures configured to be engaged with the second outer gear structures. A first intersection line is defined between each of the first inner gear structures and a sectional surface. An angle between the first intersection line and a first horizontal line is within a range from 0.1 degrees to 5 degrees.