A gear device includes: a crankshaft that has an eccentric portion; an oscillation gear that has a plurality of external teeth and oscillating and rotating due to rotation of the eccentric portion; a carrier that rotatably supports the crankshaft; an outer cylinder that has a plurality of internal teeth respectively engaged with the external teeth of the oscillation gear; and a cylindrical portion that extends through the oscillation gear, the carrier, and the outer cylinder, wherein the carrier and the outer cylinder coaxially and relatively rotate due to oscillation and rotation of the oscillation gear, and wherein the cylindrical portion is disposed in either a rotational state, in which the cylindrical portion rotates, or a stationary state, in which the cylindrical portion does not rotate, depending on relative rotation of the carrier and the outer cylinder.