A reservoir unit which is included as an element along a circulation path of a cooling system includes a cartridge and a cartridge loading unit which are configured to be removable from each other. The cartridge includes a reservoir chamber that stores a circulating liquid, and a connection portion in fluid communication with the reservoir chamber. The cartridge loading unit includes a connection receiving portion, to which the connection portion is connected, and a connection port. When the cartridge and the cartridge loading unit are attached to each other, the connection portion, the connection receiving portion and the connection port form a feed path that allows feeding the circulating liquid to the circulation path outside, and a collection path that allows collecting the circulating liquid into the reservoir chamber.