To provide a compact changeover mechanism of a planetary gear simple in structure, less in number o parts, and excellent in easiness of layout.

In this changeover mechanism of the planetary gear so formed that the rotatingly driving force of an internal gear 2 on an input shaft 1 is differentially distributed by an output from a sun gear 6 and a carrier 5 through the engagement thereof with a pinion gear 3, the internal gear 2 is formed slidably in axial direction so that the input shaft 1, carrier 5, and pinion gear 3 are selectively engaged or meshed with each other. Thus, the changeover mechanism of the planetary gear capable of selectively performing a normal differential operation, a differential lock operation, and one-way direct coupled differential operation can be provided at low cost with a smaller number of parts, simple structure, and excellent easiness is layout.