An automatic transmission that includes oil passages with a simple structure provided for twin clutches can allow the number of production steps to be reduced. The automatic transmission includes a number of gear trains equal to the number of speeds between a primary shaft and a countershaft. The automatic transmission performs connection and disconnection of a rotational driving force between a crankshaft and a gearbox by means of twin hydraulic clutches including first and second clutches, which are made of the same components and disposed on the axis of the primary shaft back to back. Oil passages can be formed into a more simple shape and can be configured to have the same total length. The first clutch performs connection and disconnection for first and third speeds, and the second clutch performs connection and disconnection for second and fourth speeds.