PURPOSE: To provide a signal processor which can change the code of a connection coefficient by the learning processes in the circuit of a neural network.

CONSTITUTION: A connection coefficient storing part 51 is provided to store the value of a connection coefficient, together with a code storing part 52 which stores the code of the connection coefficient, an up-down counter 54 which adds the increment of the connection coefficient value of the 1st code to this coefficient value and then subtracts the increment of the connection coefficient value of the 2nd code from the total coefficient value of the 1st code, a code bit storing part 55 which stores the arithmetic result of the counter 54, and an EXOR 53 which secures an exclusive OR between the output sent from the part 55 and the output sent from the part 52. Then, the code of the part 52 is changed by the output of the EXOR 53.