PURPOSE: To enable a feed amount of ink to be automatically controlled, by a method wherein light is applied on a printing surface from a light emitting device to be reflected while the printing surface is being moved, and an integration value of an electric signal converted from a light accepting amount of each photodetector is compared with a reference value to be fed back.

CONSTITUTION: A variation of a light accepting amount corresponding to a printing content is converted to an electric signal with each photodetector 18 following motion of a printing surface A, and is amplified with an amplifier D. Thereafter, it enters an integrator E, and is integrated therein. Therefore, when the motion of the printing surface A is made constant, signals having been obtained by integration within a section from a front edge to a rear edge of a sheet of a paper surface are successively outputted following the motion of the printing paper A. Then, when respective electric signals corresponding to a memory F and a memory C are taken out and put into an operation circuit H, the operation circuit H take out a difference of respective electric signals as a difference in voltage. This signal is sent to an amplified X to be amplified, and the amplified signal is sent to an ink regulator Y which supplies a specific amount of ink conforming to a measuring track. Thereby, ink can be automatically regulated by inputting data of a standard printing state as a reference.