Abstract: A fluid control system of this invention is constituted by a controller and a cylinder (21). The controller comprises: for example, a box (4) having a first chamber (18) having a fixed volume to which a fluid is supplied from a fluid pressure generating source and a second chamber (19) of a variable volume which is connected to a cylinder port (29), such chambers (18) and (19) communicating with each other by a fluid passage (2); a casing (8) engaged with the box (4) for relative movement in the direction of action of a load and having a fluid discharging passage (15) communicating with said second chamber (19) of said box (4); and a valve (5) arranged so that fluid passages (2, 3) respectively between the first chamber (18) and the second chamber (19) and between the second chamber (9) and the fluid discharging passage (15) are closed in the case of no application of a load or an external force while they get into different states in the case of application thereof.