The present invention provides a fluid controller which enables fine adjustments of low flow rates and enables a user to set desired low flow rates. Low-flow-rate piston stop-position setting means 12, which sets a position at which a low-flow-rate piston stops in upward movement thereof, includes: a low-flow-rate control screw 31 which is screwed in a through screw hole 15a provided in the top wall 15 of a casing 4 in such a way as to provide a gap between itself and the upper end surface of a valve rod 5, a tapered portion 32 provided at the tip end portion of the low-flow-rate control screw 31, a stopper piston 33 which has, in an upper surface thereof, a tapered portion 34 facing to the tapered portion 32 with a predetermined interval interposed therebetween and also has a lower surface 33a for restricting the upper position at which the low-flow-rate piston 8 stops, and a plurality of force transmission spherical members 35 which are interposed between the tapered portion 32 of the low-flow-rate control screw 31 and the tapered portion 34 of the stopper piston 33 for transmitting force therebetween.