PURPOSE: To detect the variation of two words at a high speed by using a multiplexer, a register, an EXOR group, a priority encoder, a decoder and a control circuit respectively to constitute a variation detecting circuit for two words.

CONSTITUTION: The 1st and 2nd words serve as an input at one side of a multiplexer 1 consisting of an input 100. An EXOR group 3 obtains an EXOR between an output 110 of the multiplexer 1 and an output 130 of a register 2. A priority encoder 4 supplies the output 130 and delivers an encoding output 140 corresponding to one of plural variation bits. A decoder 5 decodes the output 140 to use it as to input at the other side of the multiplexer 1. A control circuit 6 switches the multiplexer input to an output 150 from a signal 100 after receiving the 1st and 2nd words. Hereafter an encoding output is obtained for each variation bit until the register 2 has no variation bit any more.