The object of the present invention is to dissolve the shock originated from engaging and disengaging of the clutch during acceleration, when using a mesh type transmission provided with a clutch for engaging and disengaging the shift gear and the output shaft. A mesh type automatic transmission equipped with a plurality of shift gears 11, 13, 14 provided so as to rotate freely to an output shaft 5 for providing driving force to the wheels, a plurality of cogged clutches 12, 15 engaging with the output shaft 5, and a plurality of counter shift gears 8, 9, 10 corresponding to each speed, which are fixed to a counter shaft 6 rotated by the rotation of an input shaft 3 rotated from the power of an engine 43, and which mesh with the plurality of shift gears 11, 13, 14, for performing automatic shifting by controlling engagement of one of the plurality of cogged clutches 12, 15 with the voluntary shift gears of the plurality of shift gears 11, 13, 14 determined from the accelerator command value and the vehicle speed, is used. The automatic transmission is provided with an assist mechanism for transmitting the rotational force of the input shaft 3 to the output shaft 5 via the assist shaft 31 which rotates from the rotation of the counter shaft 6, when no cogged clutch of the plurality of cogged clutches 12, 15 is engaged with any of the plurality of shift gears 11, 13, 14, occurring during switching when an engaged cogged clutch of the plurality of cogged clutches 12, 15 is disengaged and one of the cogged clutch of the plurality of cogged clutches 12, 15 is to be engaged.