Chirp waves generated in a transmitting/receiving unit is supplied to ultrasonic sensors. Signals output from the ultrasonic sensors are supplied to the transmitting/receiving unit and summed in a signal processing/recording unit. The signal processing/recording unit performs mutual correlation processing between the summed signals and the chirp waves and calculates a peak generation time difference. Necessity of exchanging a pipe is determined by calculating and recording the thickness of a pipe from the calculated time difference and calculating a difference between thicknesses measured in the past and the present.