An ultrasonic flaw detecting apparatus comprises an array prove, an element-group defining circuit, a calculator, a signal receiver and a generator. The array probe comprises a plurality of piezoelectric elements, each of the plurality of piezoelectric elements being configured to transmit and receive an ultrasonic wave to and from an inspection object. The element-group defining circuit is configured to select, as an element group, plural consecutive piezoelectric elements from the plurality of piezoelectric elements, set a reference position of the element group based on array arrangement information of the plurality of piezoelectric elements in the element group and based on a weighting value of each of the plurality of piezoelectric elements in the element group, and calculate a propagation path of an ultrasonic beam from the element group based on the reference position and a predetermined refraction angle. The calculator is configured to calculate a delay time of each of the plurality of piezoelectric elements in the element group in such a manner that the ultrasonic beam is configured to propagate along the propagation path. The signal receiver is configured to receive respective ultrasonic waves received with the plurality of piezoelectric elements as detection signals. The generator is configured to generate at least one composite signal for the ultrasonic beam having the propagation path based on the detection signal and the delay time.