A method of detecting abnormality in an air-fuel ratio control system for an internal combustion engine, the air-fuel ratio control system having a proportional control valve arranged in an air supply passage for supplying secondary air therethrough to the engine and controlled in response to an output signal from an oxygen concentration sensor arranged in an exhaust system of the engine in a manner such that its valve opening varies in proportion to the magnitude of driving current supplied thereto. A circuit is provided which includes a first transistor, the proportional control valve, and a second transistor arranged in the order mentioned and serially connected with each other between a power source for supplying a predetermined voltage and ground. A value of voltage at a predetermined location in said circuit is sensed while the first and second transistors are in predetermined states of conduction and non-conduction. It is determined whether there is abnormality in the air-fuel ratio control system, on the basis of the sensed voltage value.