PURPOSE: To carry out processing of comparison of digital signals with a simple configuration of a circuit by comparing directly serial data without converting them temporarily into parallel data and regardless of the size of the data.

CONSTITUTION: A serial data A and a serial reference data B and, at the same time, the inverted signal of the data B and A are input to AND circuits 12 and 13 respectively. Then, the outputs of the circuits 12 and 13 are connected to D terminals of FFs 14 and 15 respectively. Here, outputs Q of FFs 14 and 15 are signals to show the compared result A>B, which is held until the next reset pulse is given. And if the data A is smaller than the data B, then, an FF15 is set and a signal to show the compared result A<B is output. Also, when the data and the data B are equal, neither the FF15 nor 15 is set, and the signal of the compared result A=B is output through an AND circuit 18, as the inverted output Q' of each comes to "1".