PURPOSE: To change the mesh arraying direction without using a memory device which stores the mesh concentration value, by comparing an image signal corresponding to the concentration of an original with a mesh signal which varies its peak value with each prescribed scanning frequency N and with a repeating waveform if a certain frequency.

CONSTITUTION: A continuous gradation original 2 is scanned to obtain a picture signal S0 corresponding to the concentration of the original. At the same time, a mesh signal S1 formed with a repeating waveform of a certain frequency is produced within the time corresponding to the time axis of the signal S0. The signal S1 is fluctuated periodically so as to vary repetitively its peak value with each prescribed scanning frequency N. Then the signal S1 is compared with the signal S0 to obtain a mesh image signal S2. The mesh signal can be produced by an electrical method, and as a result it is not required to store previously the mesh concentration. Thus a large-capacity memory device is eliminated.