To reduce the dimension of an automatic transmission in its radial direction and costs by arranging a multi-disk disk between the opening end parts, being arranged oppositely to each other, of two drums.

A multi-plate brake 24 is arranged between the opening end parts 26a and 28a, being arranged oppositely to each other, of two drums, that is, first and second drums 26 and 28. A first space part 40 for housing a first piston 16 and a second space part 42 for housing a second spring 22 are respectively arranged in the outer diameter sides of the first and second drums 26 and 28. Thus, according to the specifications of a machine, the first piston 16 is arranged by forming a first hydraulic chamber 12 in a part more to the left side than the multi-disk brake of a case 10. A second piston 18 is arranged by attaching a second hydraulic chamber 14 in a part more to the right side than the multi-disk brake 24 of the case 10.