PURPOSE: To adjust the outflow amount of a fluid by adjusting the opening degree of a fluid outflow port, by independently operating two kinds of piston parts and appropriately imparting the generated air pressure from three air inflow ports.

CONSTITUTION: Two separated pistons 4a, 4b and the valve stem 5, of which the leading end is processed in a tapered form, present at the extended part of the piston 4b are provided in a body main body 1 in a connected and fixed state. The valve opening degree of a valve port 3C is adjusted by the slide action of the pistons 4a, 4b and the valve stem 5 operated by air pressure. By this method, an adjusting valve performing the adjustment of a flow amount accurately and rapidly and excellent in durability is obtained.