PURPOSE: To prevent overflow of a processing liquid by external oscillation or movement of a member in a tank by providing a bypass route consisting of an auxiliary tank and stop valve to the outside of a processing liquid tank.

CONSTITUTION: The liquid developer in a developing tank 14 flows through a valve 61 and a pipe 63 into the auxiliary tank 65 when only the solenoid stop valve 61 is opened. The valve 61 is changed over to the shut position when the liquid developer is filled in the tank 65. The liquid level of the liquid developer in the tank 41 lowers down to CD2 and stops if the capacity of the tank 65 is adequately determined. The outflow and splashing of the liquid developer by the oscillation of the processing device do not arise in such low liquid level state. The liquid developer in the tank 65 is force-fed by a circulation pump 47 and is driven into the tank 41 when the pump 47 is started after the stop valve 62 is changed over to the open position while the valve 61 is held shut. The liquid level of the liquid developer in the tank 41 rises accordingly. The valve 62 is again changed over to the shut position and the liquid level in the tank 41 restores the original level when the liquid developer in the tank 65 is substantially eliminated.