The apparatus and method prevent inspection omission when ultrasonic flaw detection using a transducer array is applied to inspection of a test object being conveyed at a high speed. The method for inspecting a cross section of a test object using a transducer array comprised of many ultrasonic transducers arranged in one dimension includes the steps of transmitting ultrasonic waves from some or all of the ultrasonic transducers in the transducer array, receiving reflected waves generated by the transmitted ultrasonic waves using some or all of the ultrasonic transducers in the transducer array, converting the received signals into digital waveform signals, transforming the timing between the digitalized signal data received by each transducer element in at least one transducer group, which includes plural transducer elements selected from the transducer array, on the basis of the distances between the transducer element and spatially continuous focuses of receiving beam set to be formed in the test object; and summing all the timing-transformed signals received by transducer elements in the transducer group.