PURPOSE: To constitute a two-speed single reduction gear assembly of a landing gear for a semitrailer with fewer, simpler, and smaller parts.

CONSTITUTION: A first extensible leg mounted on the front end of the semitrailer has an input shaft 75 movable in the axial direction and an output shaft 60 connected to an input shaft of a second leg. When the input shaft 75 is pressed to be in the low speed operation position, a pinion gear 83 of the input shaft engages a middle gear 72 of the output shaft 60, so that both legs are made to make low speed operation via rotation transmitting means 74, 34, 26. On the other hand, when the input shaft 75 is pulled to be in the high speed operation position, the pinion gear 83 of the input shaft engages a spline slot 86 inside a high speed drive gear 85, and the gear 85 engages a gear 73 (smaller than the middle gear 72) of the output shaft 60, so that the legs are made to make high speed operation via the rotation transmitting means.