To solve such a problem the the filled-up state of a waste ink storage tank cannot be accurately detected for the reason of reflection from the surface of the waste ink or the like.

Brightness and chroma saturation are obtained by converting image signals R, G and B obtained by reading the surface of the waste ink 151 with a CCD sensor 152 in a waste ink storage tank 150 to the brightness index L*, and the chromatic indexes (hue) a* and b* by a three-color conversion matrix 162. When the brightness is lower and the chroma saturation is higher as compared with a threshold value, it is judged that the tank is filled up with the waste ink.