PURPOSE: To make it possible to perform highly accurate time calibration in a relatively simple constitution, by calibrating the digital clock device through a gate circuit which receives the detected signals at two points which brackets the leaking point and is operated by the signal.

CONSTITUTION: When a calibrating switch 7 is opened and a communication circuit 5 is cut, signal detecting circuits 6 and 6a, which are located at two positions on the input side and the output side bracketing the leaking point of the pipe, detects it, and contacts 10 and 10a are closed. In the case a time calibration starting switch 91 for similar clock devices 9 and 9a is turned ON for the calibration, a gate circuit 93 is opened by the detected outputs from the switches 10 and 10a. Hour, minute, and second setting time set in a time setting numeral value setter 92 is set in the digital clock 95. Then, an oscillating pulse generated in an oscillator 94 at every ms unit is applied to the clock 95. The time calibration for measuring the pressure wave propagating time is performed highly accurately at every ms unit in the relatively simple constitution.