PURPOSE: To obtain binary signals or multivalue signals that correspond correctly to the density distribution of a picture by determining reference signals by a standard white reading signal and standard black reading signal corresponding to respective positions of photoelectric conversion elements.

CONSTITUTION: An operator sets an original and reads a standard white original by reading a white original pressing plate. A processor 15 reads a standard white read command signal increasing address data of a memory 121 every time a picture element clock arrives. Then, the operator reads a standard black original signal in a memory 122 similarly with a light source turned off. When a binary processing command signal is given to the processor 15, the operator initializes read addresses of memories 12, 12, and waits for arrival of the picture element. A read signal of a photoelectric conversion element 4 is given to an A/D converter 8 as the upper limit standard and lower limit standard. The converter 8 compares the read signal with these standards and outputs density tone data.