PURPOSE: To easily detect the existence and position of a pinhole by installing sound wave detecting sensors at two places on a tubular body, detecting the sound which is formed by the leak of the pressurized gas from the pinhole on a pipe, and comparing the wave shape of the sound wave.

CONSTITUTION: When a pin-hole 11 exists on a tubular body 1, pressurized gas leaks from inside, and leak noise reaches both the edges of the pipe 1, and detected by the sound wave detection sensors 21 and 22, and the result is displayed on the display devices 31 and 32, and the sound wave shape is compared. If the wave height values of the displayed wave shapes of the sensors 31 and 32 are equal, the pinhole 11 exists at the intermediate point of the positions of the sensors 21 and 22, and if the wave height value on the display device 31 side is sufficiently large, it is judged that the pinhole 11 exists in the vicinity of the sensor 21. The installation division of the sensors 21 and 22 is represented in ratio form from the ratio of the attenuation degree of the wave form, and the position of the pinhole can be calculated.