A first container is operable to accommodate a liquid toner, and provided with a window. A reflector is disposed in the first container. A light emitter is operable to emit light to the reflector through the window and the liquid toner. A light receiver is operable to receive the light reflected by the reflector through the liquid toner and the window. A detector is operable to detect a concentration of the liquid toner based on the light received by the light receiver.