PURPOSE: To fetch efficiently a necessary signal and to make signal processing easy, by scanning linearly a photodetection surface in a prescribed angle range in association with a working point and detecting the working line in accordance with the fetched video signal.

CONSTITUTION: A video camera 11 is applied with a period signal from a frequency signal generating circuit 12 and clock pulses from a clock pulse generating circuit 13 and applies a video signal to a comparator 14 by scanning the photodetection surface around the working point in, for example, a circular shape. The comparator 14 compares a set voltage and the video signal and applies the output thereof to a gate pulse generating circuit 16. The circuit 16 applies a gate pulse as one input to an AND gate 17 in accordance with the output of the comparator 14 and the period signal. Since clock pulses are applied as the other input to the gate 17, the clock pulses corresponding to the pulse width of the gate pulse are outputted from the gate 17. Thus an arithmetic processing part 18 makes simple arithmetic processing by counting the number of the clock pulses thereby calculating the direction of the working line.