A fluid device and a fitting are connected so that fluid supply/discharge passage in the fluid device, and a fluid passage in the fitting are in communication with each other. A functional component, which allows a fluid flowing between the fluid device and the fitting to pass there though and which exerts an action on the fluid, is interposed and sealed between the fluid device and the fitting. An annular groove and an annular projection are fitted to each other and are formed in the fluid device and the fitting, respectively.