PURPOSE: To accurately measure the flow rate of the air introduced into and discharged from a patient inserted into a respirator with high sensitivity by introducing the air amount proportional to the air amount passing a main conduit to a side stream conduit.

CONSTITUTION: A main conduit 13 has a venturi 15. The small part of the air flowing via the conduit 13 is directed to pass a flow sensor 24. A side stream sensor sub-unit 16 has at least one side stream conduit 26 having considerably smaller cross section than that of the conduit 13, and one end of the conduit 26 has a positive pressure port 18 fluid communicating with the conduit 13 at the upstream of a venturi throat, and the other end of the conduit 26 has a negative pressure port 20 fluid communicating with the conduit 13 at the venturi throat. The flow rate of the air passing the conduit 26 is substantially proportional to the flow rate of the air passing the conduit 13. The sensor 24 disposed in the conduit 26 generates an output signal, and then regulates the output signal to convert it into the various air flow parameters of the patient.