Abstract: For use in a rotary printing machine comprising at least one rotary drum, an image carrier medium having an image area to be printed on an image supporting material, and a printing ink supply unit, an ink detecting device which comprises a plurality of sensor rolls rotatably supported in series on a single fixed shaft in frictional contact with the printing drum through a layer of printing ink interposed between said sensor rolls and said printing drum a first means for separately detecting rotational speed of each sensor rolls, a second means for detecting rotational speed of the printing drum, and a control means for comparing the rotational speed of each sensor roll, under the influence of the slip between the sensor rolls and the printing drum, and corresponding reference values respectively indicative of rotational speed of each sensor roll measured under such a condition that optimum quantity of printing ink is applied on the outer periphery of the printing drum, and accordingly controlling said ink sup