To linearly change the relation between flow rate and differential pressure sensitivity characteristic and easily and surely detect high and low flow rates by forming a resistor of a fixed resistor and a variable resistor.

A block 21 as a fixed resistor formed of substantially cubic plastic and an orifice 22 formed of a disc-like flexible thin film are juxtaposed in the connecting part of a lateral pair of flow pieces 1a, 1b, and a parallel plate 21c is provided on the block 21 in parallel to the flowing direction of a gas. The fluid resistance is substantially linearly changed according to the increase and decrease of the flow rate, and the differential pressure sensitive characteristic is thus also made linear, so that differential pressure can be surely detected both high flow rate and low flow rate. Consequently, when the flow rate measured on the basis of this differential pressure, the electric treatment is facilitated. Further, occurrence of a noise in a differential signal by the effect of a connecting pipe at high flow rate can be minimized.