PURPOSE: To make a minute pattern on the entire region of a linear sensor binary by using a gain bias control signal obtain by giving a specific time delay to an analog video signal subjected to sample-and-hold at a prescribed period as a threshold value.

CONSTITUTION: A floating threshold signal 8 is generated by amplifying an analog video signal 5 outputted from a CCD linear sensor 1 by an amplifier circuit 2 through a video signal delay circuit 3 and a gain bias control circuit 4. The circuit 3 consists of a sampling signal generating circuit 3-1 and sample-and- hold circuits 3-2∼3-4. The circuit 3-1 generates a sampling signal S1 of a period T and a signal 7-1 is generated from an output signal 6 of the circuit 2 by the circuit 3-2. The signal 7-2 is generated with a signal S2 delayed by a time T' (T/2<T'<T) from the signal S1, and a signal 7-3 is generated by a signal S3 delayed by the time T' further. The signal 7-3 is fed to the circuit 4 and the minute pattern of the entire region of the sensor 1 is binary-coded by using the signal 8 as a threshold value.