PURPOSE: To prevent the damage of a pinion gear through relief of the load of the pinion gear by reducing an engine output itself, inputted to the pinion gear, when the transmission mode of a planetary gear mechanism is a mode wherein a load is applied to the pinion gear.

CONSTITUTION: An automatic transmission AT comprises a torque converter 1, a multistage gear transmission mechanism 2, and a planetary gear transmission mechanism 3 for over drive arranged between the torque converter and the multistage gear transmission mechanism. The planetary gear transmission mechanism 3 comprises a planetary carrier 301 rotatably supporting a planetary gear 300, a sun gear 304 coupled to an internal gear 303 through a direct clutch 302, and a pinion gear 305 interposed between the sun gear 304 and the planetary gear 301. In this case, in the transmission mode of the planetary gear mechanism 3, when a transmission mode inputted to the pinion gear 305, securing the sun gear 304, and outputted from a ring gear is detected, control is made in the direction in which an engine output is reduced.