A transmission includes first and second input shafts connected via a first clutch to a power source, an output shaft, a first shift gear group including shift gears arranged between the first input and output shafts, a first switching mechanism including a synchronizer for selecting one shift gear from the group, a second shift gear group including shift gears arranged between the second input shaft and the output shaft, and a second switching mechanism including a synchronizer for selecting one shift gear from the group. In order to learn a synchronizer balk position, either the first or second shift gear group, where no targeted synchronizer belongs, is set to any shift gear, both the first and second clutches are engaged, the balk position of the synchronizer is detected in a process of the targeted synchronizer being slid, and information specifying the detected position is stored.