PURPOSE: To detect a phase signal stably even in the presence of a radio wave disturbance.

CONSTITUTION: A sum S of black level data in a scanning window is obtained in the step 1. The sum S is compared with a threshold level A and B respectively. In the case of S≥A, black level data are written, in the case of B<S<A, reception data are written and in the case of S≤B, white level data are written in a RAM respectively. The data written in the RAM are shifted by one picture element in the main scanning direction. When one line of the data is not yet processed, the processing is returned to the step S1. When the processing by one line is finished, a pulse width of a phase signal is measured. When no phase signal is detected, the data are shifted by one line in the subscanning direction and the processing is returned to the step S1. When the phase signal is detected, the processing is terminated.