To obtain a wet plate-making machine capable of accurately measuring toner concentration, irrespective of soiling on the light emitting surface of a light emitting element, soiling on the light receiving surface of a light receiving element and soiling on the inside wall surface of a light transmission cell in a concentration detection unit, and capable of coping with the abnormal state of the concentration detection unit.

The wet plate-making machine is provided with a spray nozzle for injecting developer on the surface of a master sheet with image information exposed, the concentration detecting unit for detecting the toner concentration in the circulating developer, and a controller to which an output signal from the concentration detection unit is inputted. Besides, the concentration detection unit is provided with the light emitting part and the light receiving part arranged opposite to each other across the light transmission cell through which the circulating developer flows. Besides, in the case of emitting the light from the light emitting part and reading the light receiving intensity, an alarm signal showing that the concentration is not normal is transmitted by the controller when the light receiving intensity read from the light receiving part is not within the specific range, and a signal from the concentration detection unit is disregarded.