PURPOSE: To facilitate the control of the amount of a liquid or the position of the liquid level of the liquid and to make it possible to keep the height of the liquid level constant even at the time of relenishment of the liquid by a method wherein a variation of the liquid level in a tank is controlled according to the amount of dipping of a dipping member, which is dipped in the liquid.

CONSTITUTION: A solder 12 decreases by a solder plating work and the liquid level of the solder descends. The drop of the liquid level is detected in order by a photosensor 13 and a float 14 is made to descend to the descent of the liquid level by a control part 22. Whereupon, the liquid level of the molten solder 12 ascends by the amount of dipping of the float 4, the photosensor 13 sends a detection output to correspond to that to the part 22 and the operation of a motor 19 is made to stop. When the float 4 is between the up-and-down intermediate position of the solder 12 and the lower limit position of the solder 12, the liquid level goes up if a solid solder is fed in a tank 10. This rise of the liquid level is detected by the photosensor 13, the control part 22 outputs a signal to rotate the motor 19 in the direction of rise of the liquid level while the motor 19 is controlled, the float 4 is made to ascend and the rise of the liquid level is suppressed. In such a way, decrease in the molten solder 12 is replenished by the float 4 and the liquid level can be kept constant.