PURPOSE: To easily cut a trouble spot at the time of maintenance and to prevent unnecessary switching from being executed by providing a logic deciding means to decide the quality of all diodes from the operating condition of the diode and the output sending condition of a transmitter.

CONSTITUTION: The quality of all the diodes in a diode switch means 4 is decided by a logic deciding means 3 from the diode operating condition, which is obtained by detecting a current to flow to the diode constituting a diode switch means 4 and an impressing voltage, and the output sending condition. Then, these deciding information are added to a switching logic means 2. In this switching logic means 2, a driving signal is sent to the diode switch means 4 with utilizing condition information, which show the operating condition of the transmitter, and these deciding information. After that, the output of the selected transmitter is sent to an external part and the output of the unselected transmitter is not sent to the external part. Thus, even when a trouble is generated the trouble spot can be easily cut and possibility to execute the unnecessary switching is improved.