PURPOSE: To enable automatic replenishment of the optimum amount of a fluid object without being affected by environmental conditions and the state of the fluid object, by detecting changes in the amount of the fluid object in a container depending on variations in the cavity resonance frequency to open or close a control valve for replenishing.

CONSTITUTION: For example, a sensor 5 comprising a sound emitting section 3 and a sound collecting section 4, a feed tube 6 and a control valve 7 are provided on the top of a container 1 holding a fluid object 2 such as liquid nitrogen to construct a container section 10 while a control section 20 is provided which comprises a control valve driving sector 11, a sound emission driving sector 12, a sensor amplifying sector 13, a frequency selecting sector 14, a point A/B frequency generating sector 15 and a decision sector 16. Then, as the amount of the fluid object 2 approaches the lower limit A, the cavity resonance frequency generated in the container 1 lowers gradually and the frequency of the cavity resonance voltage is selected with a frequency selecting sector 14, the output of which is compared with the point A resonance frequency to be inputted from the point A/B frequency generating sector 15 by the decision sector 16. When they coincide, the control valve 7 is opened to fill a charge 2 and then, closed when the upper limit B is reached.