To provide a change gear capable of automatically changing gear according to the change in the torque, gaining a great torque and enlarging the change gear range.

A first rotating body 3 having an input internal gear in the inside perimeter and rotating with an input shaft 1 is installed at an end part of an output shaft 2 of the input shaft 1. A secondary rotating body 5 is rotatable and outwards inserted in an end part of the input shaft 1 of the output shaft 2. An output gear 14 is outwards inserted and secured to an end 13 of the output shaft 2 protruded from the secondary rotating body 2 into the input shaft 1, and a first planetary gear 6 revolving around an axis L of the input shaft and the output shaft 1, 2 engaging with the output gear 14 and a secondary planetary gear 7 revolving around the axis L of the input shaft and the output shaft engaging with the internal gear 4 are rotatable and annexed to the secondary rotating body 5 around the axis parallel to the axis L of the input shaft and the output shaft respectively. The third planetary gear 15 revolving and rotating with the first planetary gear 6 is engaged with the secondary planetary gear 7.