A speed ratio switching type strain wave gearing can switch the speed ratio of output rotation with respect to one input rotation into two states or multiple states with a simple configuration. The speed ratio switching type strain wave gearing includes first and second internally toothed gears, an externally toothed gear having first and second external teeth formed on the external peripheral surface thereof, a wave generator that causes the first and second external teeth to partially mesh with the first and second internally toothed gears, a clutch mechanism that can selectively switch the first and second internally toothed gears into a fixed state. Input rotation from the wave generator can be reduced in speed at a different speed ratio and derived from the externally toothed gear by selectively switching the first and second internally toothed gears into a fixed state.