To provide a speed change mechanism for a planetary gear, small in size and large in freedom of a gear ratio at the time of an overdrive.

A first pinion carrier PC1 and a second ring gear R2 are integrally tied, and a first sun gear S1 is composed of double sun gears S1a and S1b, and one side sun gear S1b is integrally tied with a second sun gear S2, while the other side sun gear S1a inserts a reverse clutch RC, for engaging both the sun gears S1b and S1a, with an input shaft IN between. The first pinion carrier is tied with an output shaft OUT because part of the first pinion carrier is extended from between both the sun gears, thereby inserting the following: a second clutch C2, for engaging an input shaft and a second pinion carrier PC2, between them; a first clutch C1, for engaging the input shaft and a first ring gear R1, between them; a first brake B1 for fixing the rotation of the second pinion carrier in a transmission case; and a second brake B2 for fixing the rotation of the second sun gear.