PURPOSE: To cope selectively with various check systems by comparing the data so far stored with the present data or adding both data together to always store the latest error correction detecting data as well as to produce said detecting data by a ROM circuit.

CONSTITUTION: The data supplied from a data input terminal 1 is supplied to a ROM circuit 22 and compared with the error correction detecting data stored previously in a storage circuit 2 or added to said detecting data. This comparison or addition is selected by the selection signal given from a switch selecting circuit 23. Then the control is carried out to decide whether the result of comparison is equal to an odd number, an even number or the check sum data and to store the selected result of comparison in the circuit 2. The result thus obtained is stored in the circuit 2 as the latest input error correction detecting data by the synchronizing signal sent from an input terminal 25 for sample timing.