A compact rotation output device is realized by disposing an output shaft of a reduction gear transmission coaxially with an axis line of a rotating disk while simultaneously ensuring a space in a rear face of the rotating disk through which a columnar bar or wiring etc. can pass.A reduction gear transmission is provided with an input shaft having an eccentric rotation member, an external gear that has a through hole at its center and revolves orbitally while engaging with the eccentric rotation member, and an internal gear that surrounds the external gear while meshing with the external gear and allows the orbital revolution of the external gear. The number of teeth of the external gear differs from the number of teeth of the internal gear. The rotating disk is fixed to one of the external gear and the internal gear of the reduction gear transmission. The columnar bar is formed on the rotating disk, this columnar bar rotating together with the rotating disk, passing through the external gear, and extending longer than the input shaft.