PURPOSE: To provide an optical pattern detection circuit which can detect a bit pattern as it is an optical signal and which does not deteriorate the quickness of light

CONSTITUTION: The present circuit is devised utilizing the fact that 'when the bit pattern of (n) bits are added by properly variously weighting the respective bits, the added value has a unique value decided by the content of the bit pattern. This circuit is constituted so as to uitlize it.' By a weighting means 20 having optical circuit constitution, the respective bits of the inputted optical signal 250 are weighted. By an addition means 21 having the optical circuit constitution, the weighted respective bits (optical signals) 261-264 are added. By a comparison means 22 having the optical circuit constitution, it is compared and decided whether the power of the added optical signals (AL timing of 270) is the optical power peculiar to the bit pattern desired to be detected or not.