To measure the amount of leakage quickly and accurately by discharging a gas corresponding to a discharge flow rate that is recorded when measuring a differential pressure into a master chamber.

A control part 15 closes a valve V8 and prevents suction to a mounting tool 2, closes valves V3 and V4, and opens V1, V2, and V5-V7. A master chamber 1, an inspection object, and the inside of the mounting tool 2 are evacuated by opening the valves V1 and V2. Then, the control part 15 reads the indicated pressure of a pressure gauge 9 via an I/O 16, judges whether a measurement starting preparation pressure is appropriate or not, opens the valves V3 and V4 when the preparation pressure is reached, and then instructs an operator to close the valves V5 and V6. The procedure is used to stop the swing of a differential pressure meter 4 quickly. Then, the control part 15 stands by until the instructed pressure of the pressure gauge 9 reaches a set pressure and closes the valves V1 and V2 when the set pressure is reached and then stops exhaust. The control part 15 reads a differential pressure DP1 that is indicated by the differential pressure meter 4 and a differential pressure DP2 after a set time passes. A part 14 for calculating the amount of leakage calculates and output the amount of leakage from the differential pressures DP1 and DP2 based on a specific expression.