A mesh type automatic transmission is equipped with a plurality of shift gears arranged to rotate freely to an output shaft for providing driving force to the wheels. Cogged clutches 12, 15 engage with the output shaft. Counter shift gears corresponding to each speed are fixed to a counter shaft rotated by the rotation of an input shaft and mesh with the shift gears for performing automatic shifting by controlling engagement of one of the cogged clutches with the voluntary shift gears determined from the accelerator command value and the vehicle speed. An assist mechanism transmits rotational force of the input shaft to the output shaft via the assist shaft which rotates from the rotation of the counter shaft when no cogged clutch of the plurality of cogged clutches is engaged with any of the plurality of shift gears occurring during switching when an engaged cogged clutch of the plurality of cogged clutches is disengaged and one of the cogged clutch of the plurality of cogged clutches is to be engaged.