An automatic transmission apparatus includes a transmission mechanism, an actuator arranged to rotate a shift drum, and a control device. The control device includes a determining unit arranged to detect a state of a straddle-type vehicle and to determine whether the straddle-type vehicle is in a first state in which an engaging force between the transmission gears is estimated to be equal to or greater than a predetermined value or in a second state in which the engaging force is estimated to be less than the predetermined value; a first gear shifting control unit arranged and programmed to control the actuator so that, if in the first state, the rotational speed of the shift drum becomes a first rotational speed; and a second gear shifting control unit arranged and programmed to control the actuator so that, if in the second state, the rotational speed of the shift drum becomes a second rotational speed that is lower than the first rotational speed.