An ultrasonic inspection apparatus includes a control device 12 that outputs a pulse signal for making an ultrasonic probe 11A transmit an ultrasonic wave to the ultrasonic probe 11A and that is supplied with a waveform signal obtained by converting the received ultrasonic wave from an ultrasonic probe 11B, and that computes a thickness reduction depth of a pipe 1 between the ultrasonic probe 11A and the ultrasonic probe 11B. The control device 12 computes the thickness reduction depth of the pipe 1 by selectively using a reception time of the ultrasonic wave transmitted from the ultrasonic probe 11A, reflected once by an inner surface 3 of the pipe 1, and received by the ultrasonic probe 11B, and a reception time of the ultrasonic wave transmitted from the ultrasonic probe 11A, reflected twice by the inner surface 3 of the pipe 1, and received by the ultrasonic probe 11B.