In a controller for an AC electric train which can perform powering and regenerating operations and can detect any power failure on AC overhead wires for supplying the AC power, detection of any powerfailure on AC overhead wires is carried out quickly without fail. A power failure detector (14) provided on an electric train controller (8) includes a specific frequency signal arithmetic operationsection (16) which extracts a current component corresponding to a specific frequency setting value (26) from a main transformer output current detection signal (7) to output the extracted signal as aspecific frequency current signal (18), a subtracter (19) which subtracts the main transformer output current detection signal (7) from the specific frequency current signal (18) to output the subtraction result as a current deviation (20), and a power failure detection section (23) which compares the current deviation (20) with a specified power failure detection setting value (22) and outputs apower failure detection signal (15) when the current deviation (20) is greater than the power failure detection setting value (22).