When a ringlet with a high frame transfer quality is selected as a switching candidate for each station of a transfer destination from information on frame transfer quality for each ringlet, when it is determined that a fundamental ringlet is in a preceding stage of degradation of the frame transfer quality from information on the frame transfer quality of the fundamental ringlet, and if the fundamental ringlet is different from the switching candidate, a ringlet for transferring a frame to a station of the transfer destination is switched from the fundamental ringlet to the switching candidate.