An apparatus for controlling an automatic transmission is disclosed. A 2nd speed is established in which hydraulic pressure is supplied only into an engaging chamber of a 2-4 brake by a first duty solenoid valve so that the brake is applied. A 3rd speed is established in which the hydraulic pressure is supplied into a releasing chamber of the brake and a hydraulic chamber of a 3-4 clutch by a second duty solenoid valve under the condition of the hydraulic pressure being supplied into the engaging chamber of the brake so that the brake is released and the 3-4 clutch is engaged. When the 2nd speed is changed to the 3rd speed, a controller increases the hydraulic pressure supplied into the releasing chamber of the brake and the hydraulic chamber of the 3-4 clutch in a case where a failure, in which the first solenoid valve is fixed under a condition that the first solenoid valve supplies the hydraulic pressure into the engaging chamber, is detected.