Abstract: Gate switches can switch an amount of attenuation, and are set in a distributed manner on distribution lines and a trunk on subscribers' house side in a CATV transmission path having a tree structure with an optical node that follows a headend being taken as a starting point. An upstream-transmission-quality monitoring unit monitors an S/N ratio of an upstream signal obtained from an upstream port having the headend connected thereto to detect a decrease in upstream transmission quality based on a degree of decrease and a continuation state of an S/N ratio. When the upstream-transmission-quality monitoring unit detects a decrease in upstream transmission quality, a noise-generation-source searching unit performs a sequential switching control over the amount of attenuation at the gate switches provided on the CATV transmission path from upstream to downstream to search for a source of generation of upstream ingress noise.