PURPOSE: To measure air leakage simply and in a short time by obtaining speed of pressure drop of an air system and its proportional constant of pressure and performing calculation.

CONSTITUTION: In order to measure air leakage of an air system 3a the use of an air equipment 4a is stopped and the air 3a is shut off by closing a valve 5a. After this leakage coefficient is calculated by means of a measuring instrument 6 and input to a display section 10. When the normal pressure of the air system 3 is known, the pressure of the system 3a is measured for once with the time from closing the valve 5a to the time of measurement taken as ΔT. If the normal pressure of the air system 3a and its volume and other required information is input to the computer of the measuring instrument 6, air leakage is automatically calculated and the results are outputted to the display section 10. By inputting the consumed electric power of a compressor 1 and so on to the computer 11, the amount of electric power loss of the air leakage is calculated and displayed on the displayed section 10.