PURPOSE: To detect the quantity of offset preventing solution by applying upward force to an application roller when the application roller is descended by its tare weight at the inoperative time of the application roller and separated from a fixing roller and stopping the application roller at the height corresponding to the liquid level in a container.

CONSTITUTION: The application roller 3 is arranged adjacently to the fixing roller 1. At the time of copying, a solenoid 10 is actuated, a press lever 9 is rotated upward, the roller 3 is lifted and contacted with the roller 1 to supply the offset preventing solution. At the end of copying, the solenoid 10 is made inoperative, the roller 3 is descended by its tare weight and the lever 9 is rotated downward. In accordance with the downward rotation of the lever 9, a detecting lever 13 is also rotated downward and engage with a pin 19 of an auxiliary lever 16 and the excitation of a spring 17 is transmitted to the roller 3 through the lever 13, a shaft 8 and the lever 9, so that upward force is applied to the roller 3 and the roller 3 stops at a descending position corresponding to the liquid level in the container. A photosensor 14 can detect that the offset preventing solution is in a lack position.