PURPOSE: To reduce the size and cost of the multiple converter of doubled constitution by monitoring respective constitution parts of the multiple converter by utilizing the code converting function that the stand-by system of the multiple converter has.

CONSTITUTION: A path A(B) is set and the high-order group signal output of an in-use multiple converter SYS is inputted to the stand-by multiple converter SBY. Then the output signals of bipolar/unipolar code converting circuits 11∼1n(71∼7n) and respective input signals corresponding to unipolar/bipolar code converting circuits 7'1∼7'n are compared with each other by a testing circuit 9. Paths C1∼Cn are set so as to monitor the operation states of the unipolar/bipolar code converting circuits 71∼7n. Then the testing circuit 9 compared the input signals of the unipolar/bipolar code converting circuits 71∼7n with the output signals of the bipolar/unipolar code converting circuits 1'1∼1'n. Consequently, all constituent circuits of the multiplexer SYS can be monitored.