In an apparatus for determining failure of a double-clutch automatic transmission having a line pressure-regulating valve, pressure-regulating valves and select valves, it is determined whether the pressure regulated by the line pressure-regulating valve is equal to or greater than a predetermined pressure and it is determined that the line pressure-regulating valve has failed if the pressure regulated by the line pressure-regulating valve is determined to be less than the predetermined pressure, whereas it is determined that one from among the pressure-regulating valves and select valves has failed if the pressure regulated by the line pressure-regulating valve is determined to be equal to or greater than the predetermined pressure.