NOVELTY - The valve (10) has an aperture part (15) arranged in a flow path between outflow-side chambers (25) connected to an outflow part (13) of an inflow side chamber (20). A valve main body part (11) is provided with a valve seat (16) arranged in each outflow-side chamber. A middle transmission part (80) engages a connection chamber (70). A valve body (38) is moved along back and forth direction with respect to the valve seat by forward and return movement of a diaphragm (30) by pressure fluctuation before and behind the aperture part and another diaphragm (35).    USE - Constant flow rate valve i.e. apparatus for fixing circulation flow volume of various to-be-controlled fluid, for use in a semiconductor manufacture field and a fuel cell. Uses include but are not limited to ultrapure water, hydrofluoric acid, hydrogen-peroxide solution, ammonia water and hydrochloric acid.    ADVANTAGE - The valve body is moved along back and forth direction with respect to a valve seat by forward and return movements of the diaphragm by pressure fluctuation before and behind the aperture part and another diaphragm such that unexpected contact of the valve seat and the valve body can be suppressed and purity degree of the to-be-controlled fluid in a higher state can maintained. The end of the transmission part is engaged with a rear end of a movable part by an inclined surface such that fluctuation to another movable part from the former movable part by the transmission part can be transmitted smoothly. The diaphragm can be biased by a pressurizing part i.e. spring, such that cost of the valve can be reduced and structure of the value can be simplified.    DESCRIPTION OF DRAWING(S) - The drawing shows a side sectional view of a constant flow rate valve.    Constant flow valve (10)    Valve main body part (11)    Outflow part (13)    Aperture part (15)    Valve seat (16)    Inflow side chamber (20)    Flow path between outflow-side chambers (25)    Diaphragms (30, 35)    Valve body (38)    Connection chamber (70)    Middle transmission part (80)