To suppress circuit scale, then improve confidentiality of communication data and prevent circuit internal operation from being analyzed and deduced, in an unauthorized manner.

In transmission from a transmission-side device 10 to a reception-side device 20, an encoder 11 makes data X0 in input data into external data Bi of initial value, sets EXOR of the external data Bi and the input data X0 into external data B0, sets EXOR of the input data X0 and X1 as external data B1, sets EXOR of input data X1 and X2 as external data B2, performs similar arithmetic to an nth bit thereafter and outputs the external data Bi and B0 to Bn-1. A decoder 21 sets EXOR of the external data Bi of the initial value and B0 as output data Y0, sets EXOR of the output data Y0 and the external data B1 as output data Y1, sets EXOR of the output data Y1 and the external data B2 as output data Y2, performs similar arithmetic to the nth bit thereafter, and outputs the output data Y0 to Yn-1.