This invention provides a fluid mechanism that can arrange a plurality of fluid device units and external fluid devices mounted as a set on the fluid device units effectively and compactly. Each of the fluid device units is arranged with respective side surfaces in a longitudinal direction of the fluid device unit tightly attached, and the external fluid devices are arranged side-by-side external to and outside of the fluid device unit in the width direction. Furthermore, as for an introducing path and a discharging path that connect the external fluid devices and the fluid device unit, the introducing path, which is short, is connected to the discharging path, which is long.