The present invention provides an in-wheel reduction gear and a wheel unit. The in-wheel reduction gear comprises a sun roller configured to rotate around a spin axis taken as a center; a plurality of planetary mills arranged around the sun roller; a circled inner ring configured to contact with a plurality of planetary mills; a casing surrounding the inner ring; a plurality of planet carrier pins configured to support each planetary mill to rotate; wheels fixed at the ends of the plurality of planet carrier pins; and a forcing mechanism configured to press the inner ring towards the planetary mills. Each of the planetary mills is capable of autoroatation while revolution around the spin axis taken as the center through contacting with the sun roller and the inner ring and receiving power from the sun roller, and the in-wheel reduction gear also has a releasing mechanism configured to pull open the inner ring and the planetary mills.