The utility model provides an automatic transmission. Therein, the second and the third clutches(C2, C3) are arranged in the axis direction and are put in an approximately neat order. The back side of the second clutch(C2) and the front side of the third clutch(C3) are opposing to each other. The first clutch(C1)is arranged at the inner diameter side of the second clutch and third clutch. In this way, the bulges(37b)of the clutch bulges(37) of the second and the third clutches(C2, C3) extend in the axis direction and are in common use. Besides, the second clutch(C2)conducts the clutch from the front side in the axis direction through the second piston(47) of the second oil pressure mechanism. The third clutch(C3)conducts the clutch from the back side in the axis direction through the third piston(53) of the third oil pressure mechanism. Therefore, a compact disposition of the first to the third clutches(C1-C3)is formed and these clutches can work individually. The utility model provides an automatic speed changer, wherein a second clutch C2 and a third clutch C3 are assembled to be arrayed orderly in an axial direction, the back side of the second clutch C2 and the front side of the third clutch C3 are opposite, and the inner diameter sides of the second clutch C2 and the third clutch C3 are assembled with first clutches C1. Thus, drum parts (37b) of clutch drums (37) of the second clutch (C2) and the third clutch (C3) are extended in an axial direction to be used together, the second clutch C2 is clutched from the front side in the axial direction by a second piston (47) of a second oil pressure mechanism, and the third clutch C3 is clutched from the back side in the axial direction by a third piston (47) of a third oil pressure mechanism. Thus, the compact assembly of the first clutch C1, the second clutch C2 and the third clutch C3 is formed, and the clutches can operate independently.