PURPOSE: To reduce fluctuation of capacity of a compressor by operating the compressor at an allowable lower limit value of capacity and setting a dead zone with respect to a detection level to adjust the capacity of the compressor when the operation of town gas supply facility is started in a level adjusting device of a water sealed gas holder in the town gas supply facility and the like.

CONSTITUTION: In a town gas supply facility which receives gas from a gas generator 1 by a water sealed gas holder 2 temporarily and then raises pressure of the gas by a first and a second compressors 3A, 3B to supply the gas, capacity of the second compressor 3B is increased up to an allowable lower limit value by an adjuster 6 after the second compressor 3B is started. At this time, if a level of the water sealed gas holder 2 which is shown by a level gauge 5 exceeds the upper and lower limit values of a dead zone which is set with respect to the fluctuation of level with margin in advance and fluctuates continuously, the capacity of the second compressor 3B is changed so far as the capacity does not reach the allowable lower limit value. When reverse fluctuation is started after the detected level reaches a peak, the capacity of the second compressor 3B is changed with margin of change which is smaller than before as a limit.