To provide a fault self-detecting type optical node device capable of detecting a fault without having any adverse effect on full-duplex communication.

The device is provided with a light shielding means 21 to become transparent and opaque, fault detection optical paths 12, 14, 15 and 16 for guiding optical signals outputted from an E-O(Electric-Optical) converter 42 to an O-E(Optical-Electric) converter 41 through the means 21, a voltage applying means 51 for switching the means 21 to be transparent or opaque, a signal comparing means 54 for transmitting an electric signal to the converter 42 when the means 21 is transparent to compare the transmitted signal with an electric signal converted by the means 41, and a fault detecting operation control means 53 for judging the presence/absence of a fault on the basis of the result of comparison. In this device, when a fault is absent, the means 21 is switched to be transparent. If no fault is detected, a bi-directional communication can be made stably while keeping the means 21 to be transparent.