PURPOSE: To prevent a malfunction by providing a comparator and a relay so as to disconnect a data transmission equipment to which power was turned off before a logic section of the data transmission equipment has an error.

CONSTITUTION: An optical switch 1 connected to optical signal transmission lines L, L' and switching a path inputting/outputting an optical signal to the data transmission equipment 2, a comparator 3 detecting the voltage drop of a power supply of the data transmission equipment 2 and a relay 5 controlling the connection between the optical switch 1 and the power supply 22 of the data transmission equipment 2 by the output of the comparator 3 are provided to a system. Through the constitution above, when the power supply 22 of the data transmission equipment 2 is turned off and the voltage is lowered, the voltage drop is detected by the comparator 3 and the connection between the optical switch 1 and the power supply 22 of the data transmission equipment 2 is opened before the logic section 21 of the data transmission equipment 2 malfunctions by the output of the comparator 3 to prevent the malfunction.