PURPOSE: To obtain an air arithmetic unit available for multi-plication-division simultaneous arithmetic by providing two methods of displacing parallel beams and two displacement detcting methods and by applying one detection output to the displacing method while regarding the other as an arithmetic output.

CONSTITUTION: To bellows 11 and 12, parallel beams 13 and 14 are connected and to force points 15 and 16 at their tips, focus beams 17 and 18 are linked. Beams 17 and 18 are linked together by coupling bar 19 and bellow 20 is connected to the extention point of it. Beams 13 and 14, on the other hand, are provided with bellows 22 and 23. Nozzles 24 and 25 are provided in the opposite direction of bellows of beams 17 and 18 so as to supply air Ps into both the nozzles. The displacement of beams 13 and 14 are detected by nozzles 24 and 25 and back pressures of beams 13 and 14 are led to bellow 20 to tilt beams 17 and 18 against spring 21 by the same angle. The back pressure of nozzle 24 is amplified by pilot valve 28 together with air Ps and then led in bellow 11, so that variation in air Ps will be output Po.