Automatic transmission with electronic control unit

In an electronic control unit, a transmission control module (2) for controlling an automatic transmission and a position sensor for detecting the range position of the automatic transmission are built in a common unit case (10). A first member and a second member to be fitted at the time of mounting the electronic control unit on an automatic transmission case are directed in the mounting direction when mounted on the automatic transmission case, and a third member is mounted on those two members. A predetermined clearance is formed at a predetermined portion on themounting path between the automatic transmission case and the third member. When the electronic control unit is mounted on the automatic transmission case, therefore, the clearance on the mounting path inbetween absorbs the positional deviation between the first member and the second member thereby to facilitate the mounting work. Thus, the mounting workability of a position sensor (3) built-in electronic control unit on an automatic transmission is improved.