A speed reducing device according to one aspect of the present disclosure includes: a plurality of speed reducer units arranged in parallel; and an output rotating body that rotates by power received from the speed reducer units. Each of the speed reducer units includes a casing and a speed reducing mechanism unit. The casing has a peripheral wall with inner teeth disposed on an inner periphery of the peripheral wall. The speed reducing mechanism unit meshes with the inner teeth to decelerate input rotation. At least two of the speed reducer units are disposed adjacent to each other such that outer surfaces of peripheral walls of casings thereof are in contact with each other. In at least one of two peripheral walls contacting with each other, a thickness of a contact portion is smaller than a thickness of other portions in a circumference of the same peripheral wall.