PURPOSE: To provide a linearity compensating circuit where a characteristic curve which is large in the free degree of the input/output characteristic curve and also smooth with a small circuit scale obtained.

CONSTITUTION: A pair of diodes D3 and D4 where one ends are coupled and input differential current is supplied to the respective other ends, an amplifying means 21 permitting a differential voltage obtained from the respective other ends of the pair of diodes D3 and D4 to be constant number-fold and a differential circuit 22 consisting of the pair of transistors Q5 and Q6 where the differential voltage obtained from the amplifying means 21 is supplied to respective bases, emitters are mutually connected and differential current is outputted from a collector are provided in a non-linear inputting and outputting circuit 19. A linearity compensating circuit is constituted in such a way that the plural circuits 19 are serially connected between input differential current supply ends 18 and differential current output ends 25 and the respective input dynamic ranges and gain of the non-linear inputting and outputting circuits 19 are made different so as to obtain a required input/output characteristic.