<p>A pressure type flow rate control apparatus (1) controls flow rate of a fluid in an environment where a ratio of P2/P1 between an absolute pressure P1 at an upstream side of an orifice and an absolute pressure P2 at a downstream side of the orifice is maintained at a value less than about 0.7. The apparatus comprises: a plate for forming the orifice (5); a control valve (2) positioned upstream of the orifice; an orifice corresponding valve (9) positioned downstream of the orifice (5); a primary pressure detector (3) positioned between the control valve (2) and the orifice (5); a secondary pressure detector (11) positioned downstream of the orifice (5); a calculation control device (6) for calculating flow rate Qc on the basis of the measured pressure P1 of the primary pressure detector (3) by a formula Qc = KP1 (K being a constant) and for outputting as a control signal Qy a difference between a flow rate command signal Qs and the calculated flow rate signal Qc to a drive unit (14) of the control valve 2; and a pressure comparing, calculating apparatus (10) for calculating the ratio of P2/P1 between the detected pressure P1 of the primary pressure detector (3) and the detected pressure P2 of the secondary pressure detector (11). The pressure P1 upstream of the orifice is adjusted by opening and closing the control valve by the control signal Qy, thereby controlling the flow rate downstream of the orifice.</p>
