PURPOSE: To improve an S/N ratio by laminating photoconductive layers having different absorption coefficients to the wavelength of incident beams and discriminating colors by utilizing changes in response to the wavelength of beams of outputs from optical signals generated in each layer.

CONSTITUTION: A chromium thin-film is formed on an insulating glass substrate 1, and divided into predetermined pattern shapes, thus forming chromium electrodes 2 as separate electrode. An amorphous silicon hydride germanium layer 3 is shaped as a first photoconductor layer, and an indium oxide tin electrode 4 is formed as a light-transmitting common electrode. An silicon oxide layer 5 as a blocking layer is applied. An amorphous silicon hydride layer 6 is shaped as a second photoconductive layer, and an indium oxide tin film is applied, and divided into separate electrode having prescribed shapes and formed. Accordingly, beams on the short wavelength side are detected in a sensor group S2 while beams on the long wavelength side are detected in a sensor group S1, and these detecting signals are arithmetically processed, thus correctly reading color drafts.