A gear device is provided with a crankshaft including a shaft main body and an eccentric portion, a linking member including external teeth and configured to transmit the rotation of the input gear to the crankshaft, a carrier configured to rotatably support the crankshaft, an oscillation gear oscillating and rotating, and an outer cylinder including internal teeth engaged with the oscillation gear. The carrier and the outer cylinder are configured to concentrically relatively rotate according to the rocking rotation of the oscillation gear. The linking member is configured to selectively take a first position where the external teeth are engaged with a first input gear and a second position where the external teeth are engaged with a second input gear.