The present invention provides a switch device for an optical signal which has high extendibility and which can suppress an increase of loss in extension. A switch device in one embodiment of the present invention includes, in a housing, four splitter portions, four switch portions, and extension splitter portions. The splitter portions are connected to path-side ports, the switch portions are connected to client-side ports, and the splitter portions and the switch portions are connected to one another via shuffle fiber arrays. One ends of the extension splitter portions are connected to path-side extension ports, and the other ends thereof are connected to client-side extension ports. The number of wavelengths processable by the switch device can be easily changed by changing a connection state of optical fibers to the path-side ports, the client-side ports, the path-side extension ports, and the client-side extension ports.