To provide a device for removing an image forming material from an image carrier, capable of preventing deterioration in performance such as peelability and toner fixability at the time of reforming an image by controlling the concn. of an unstabilizing liquid.

The unstabilizing liquid in a first liquid tank 61 is circulated/ supplied to a concentration sensor 71, to detect the concn. of the unstabilizing liquid by using the refraction of light occurring on the interface between the plane of a prism and a liquid. When the detected value exceeds a previously set reference value, the supply of the unstabilizing liquid to the concentration sensor 71 is stopped and water 76 in a second liquid tank 77 is supplied to the first liquid tank 61 from a water supplying pipe 79 through the concentration sensor 71 and a flowmeter 81 by a third pump 78.