PURPOSE: To detect defective printed matter based on dampening water supply failure and rapidly exclude the detected matter by providing a light emitting unit for emitting injecting water near the injection port of a water supply nozzle, a light receiver for receiving the reflected light from the water, and dampening water presence or absence detecting means for judging the supply or the water based on the signal from the receiver.

CONSTITUTION: The many dampening water injection nozzles 21 of a dampening water supply unit 6 are provided at substantially constant interval. A set of a light emitting unit 8 and a light receiving 14 of a dampening water detector are set at each one nozzle. Modulators 9 are gathered at each unit 8, and the outputs of the receives 14 are gathered at a controller 16, and the controller 16 judges which nozzle is normal or not. If any one is defective, a failure discharge unit may be operated, or may be operated if several of the nozzles are defective by previously setting it. As the emitting unit, the light source of a visible light such as a halogen lamp may be used.