A multistage automatic transmission comprises first and second clutches and a throttle open degree sensor. The first and second clutches are arranged to establish odd and even speeds of the transmission when engaged. A control unit of the transmission is configured to carry out determining a desired speed of the transmission upon receiving information signals; causing at least one the first and second clutches to be engaged to establish the desired speed; and applying a limitation to an engaging force produced by at least one of the first and second clutches when the throttle open degree sensor senses that the open degree of a throttle valve is smaller than a predetermined degree.