In an apparatus for detecting abnormal hydraulic pressure of an automatic transmission having first and second input shafts that input rotation of an engine through first and second clutches and an idler shaft that inputs the rotation through a third clutch, and equipped with a hydraulic pressure supply circuit having a first pressure-regulating valve that generates line pressure, a line pressure-regulating valve that controls operation of the first pressure-regulating valve based on a line pressure command value, and first, second and third hydraulic pressure detectors that detect the hydraulic pressures of first, second and third oil passages respectively, it is determined whether abnormality that makes the high pressure abnormally high has occurred in the line pressure-regulating valve based on the hydraulic pressures detected by at least one of the first, second and third hydraulic pressure detectors and a line pressure command value, when predetermined operating conditions of the vehicle are satisfied.