PURPOSE: To improve the efficiency percentage, by forming an amount of compensation write written on a sensitivity compensating layer on a photodetecting position of each unit sensor and performing compensating write while scanning each unit element, for compensating the variance in the sensitivity of sensors after manufacturing stage.

CONSTITUTION: When a retrieval signal A is inputted, a very short luminous amount for check obtained via a multiplexer MX17 and an amplifier 11 is given to a unit sensor through a write layer not exposed of a sensor array 5 via a laser unit 6. A signal detected from the array 5 is sampled and held 13 and the result of this signal obtained by comparing it with a reference signal 14 at a comparator 15 is given to a retrieval circuit 16. The circuit 16 indexes the result of a table prepared in advance at the transmission of the write layer not exposed and the photosensitivity of the write layer in response to the amplitude of the signal as the result of comparison and sets an illuminating time of the unit 6 to write it in the write layer. In this case, the MX17 passes this signal when a mode FF18 is set. When the write is finished, the register 9 is advanced, the next address is designated, a motor 8 drives a scanning mechanism 7 to write signal on the next not-exposed sensitive layer.