PURPOSE: To allow accurate inspection of underground gas leak by sustaining the pressure of gas in a sampling space at a constant level when the gas to be inspected is sampled from the sampling space through suction.

CONSTITUTION: The gas leak inspection system 9 comprises a gas injection unit 10 and a sampling unit 11 wherein the gas injection unit 10 is provided with front and rear water packers 16, 12 such that an injection space 52 can be formed in a first underground hole 6 and means 10b for injecting tracer gas 61 is disposed in the injection space 52. The sampling unit 11 is provided with front and rear air packers 31, 27 such that a sampling space 53 can be formed in a second underground hole 7. The gas leak inspection system 9 further comprises means 11b for sampling gas 61 to be inspected from the sampling space 53 through suction, means 51 for measuring the pressure in the sampling space 53, and means 11c for feeding supplementary air 63 to the sampling space 53.