An automatic transmission that changes a rotation speed of a main shaft, to which a torque from a power source is transmitted, and transmits the rotation of the changed rotation speed to a driveshaft, the automatic transmission comprising: a plurality of dog clutches (C1-C6) that each include a drive dog (53, 54) provided on the main shaft or on the driveshaft so as to be in constant synchronous rotation with the main shaft, and a driven dog (54, 53) provided on the main shaft or on the driveshaft so as to be in constant synchronous rotation with the driveshaft; a switching mechanism configured to cause the drive dog (53, 54) and the driven dog (54, 53) of each of the dog clutches (C1-C6) to approach and separate from each other to engage and disengage the drive dog (53, 54) and the driven dog (54, 53) of each of the dog clutches (C1-C6) to switch gears (Ma, Mb, Mc, Md, Me, Mf; Da, Db, Dc, Dd, De, and Df) transmitting a driving force between the main shaft and the driveshaft; a clutch switchable between a connected state, in which the torque is transmitted sliplessly from the power source to the main shaft, and a disconnected state, in which a portion of the torque is slippingly transmitted from the power source to the main shaft or in which the torque is not transmitted from the power source to the main shaft; an actuator unit (51, 52) for actuating at least the switching mechanism among the clutch and switching mechanism; and a control unit, characterized in that the control unit stores information concerning the gear stages, the control unit comprises a gear-change control means configured to control a gear-change operation, including a dog engaging operation of making the actuator unit (51, 52) actuate to engage the drive dog (53, 54) and the driven dog (54, 53) of one of the dog clutches (C1-C6), based on a relative rotational position of the drive dog (53, 54) and the driven dog (54, 53) to be engaged, the automatic transmission is arranged to transmit the driving force of the power source that is input into the main shaft to a drive wheel via the driveshaft, and the gear-change control means is configured to determine the relative rotational positions of the drive dogs (53, 54) and the driven dogs (54, 53) of the plurality of dog clutches (C1-C6) based on a rotational angle or a rotational speed of any one of a plurality of rotating shafts (46, 47) disposed in a driving force transmission path from the power source to the drive wheel and the information concerning the gear stages. a plurality of dog clutches (C1-C6) that each include a drive dog (53, 54) provided on the main shaft or on the driveshaft so as to be in constant synchronous rotation with the main shaft, and a driven dog (54, 53) provided on the main shaft or on the driveshaft so as to be in constant synchronous rotation with the driveshaft; a switching mechanism configured to cause the drive dog (53, 54) and the driven dog (54, 53) of each of the dog clutches (C1-C6) to approach and separate from each other to engage and disengage the drive dog (53, 54) and the driven dog (54, 53) of each of the dog clutches (C1-C6) to switch gears (Ma, Mb, Mc, Md, Me, Mf; Da, Db, Dc, Dd, De, and Df) transmitting a driving force between the main shaft and the driveshaft; a clutch switchable between a connected state, in which the torque is transmitted sliplessly from the power source to the main shaft, and a disconnected state, in which a portion of the torque is slippingly transmitted from the power source to the main shaft or in which the torque is not transmitted from the power source to the main shaft; an actuator unit (51, 52) for actuating at least the switching mechanism among the clutch and switching mechanism; and a control unit, characterized in that the control unit stores information concerning the gear stages, the control unit comprises a gear-change control means configured to control a gear-change operation, including a dog engaging operation of making the actuator unit (51, 52) actuate to engage the drive dog (53, 54) and the driven dog (54, 53) of one of the dog clutches (C1-C6), based on a relative rotational position of the drive dog (53, 54) and the driven dog (54, 53) to be engaged, the automatic transmission is arranged to transmit the driving force of the power source that is input into the main shaft to a drive wheel via the driveshaft, and the gear-change control means is configured to determine the relative rotational positions of the drive dogs (53, 54) and the driven dogs (54, 53) of the plurality of dog clutches (C1-C6) based on a rotational angle or a rotational speed of any one of a plurality of rotating shafts (46, 47) disposed in a driving force transmission path from the power source to the drive wheel and the information concerning the gear stages.