To enable the smooth motion of a flexible gear while fastening the flexible gear to the output side.

A drive unit comprises: a fixed-side internal gear section 25 of a housing 3; an elliptical flexible gear 5 partially engaged with the fixed side internal gear section 25 in the circumferential direction and bent in the radial direction so as to be able to change the engagement position; a planetary ring 7 inscribing the flexible gear 5 and changing the inscribed position so as to change the engagement position of the flexible gear 5; and a sun ring 9 circumscribing the planetary ring 7 and transmitting the input rotation to the planetary ring 7. The flexible gear 5 is statically joined to the output-side rotary member 11 via a deformation transit section 10 having a circumferential shape.