PURPOSE: To improve air-filling precision, by a method wherein the inner pressure of a tyre is measured, and based on the measurements, a differential pressure between the measurements and a specified pressure, a time for which the air is filled to cover a minus differential pressure, or a time for which the air is bled to eliminate a plus differential pressure, are computed to control operation to fill with the air.

CONSTITUTION: In air-filling operation, after an air chuck 20 is firstly attached to a valve 25 of a tyre 24, a ball valve 16 is opened, and a solenoid valve 19 is switched to an upper position or an air feed position. This causes the air, fed from a hydraulic source 15 through a filter 17 and a pressure reducing valve 18, to be momentarily fed to the tyre 24 by a specified amount, and thereafter, causes the solenoid valve 19 to be temporarily forced into a closed condition. In which case, an inner pressure PX of the tyre is measured by a pressure converter 21, a differential pressure between a measurement PX and a control pressure P0 is found by a computing controller part 23, and based on the differential pressure, an air-filling time ΔT is calculated. According to a computing result, the solenoid valve 19 is switched to an air feed position again.