First and second control circuit sections that mutually communicate via a series-parallel converter comprise first and second adder-subtracter respectively. When any receiving error occurs in each control circuit section, a variation value 3 is added to the adder-subtracter on the receiving side. When data is normally received, a variation value 1 is subtracted from the adder-subtracter. Initial value of the adder-subtracter is set to 9. When a current value exceeds 11, first and second error detection signal is generated to carry out alarm display or initialization, and initialization and restart of the other-side control circuit section.