PURPOSE: To enable measurement continuously for a long time, by connecting a fluid supply source between tip openings of first and second tubes and a differential pressure measuring device respectively through an orifice.

CONSTITUTION: In a Pitot tube 11, a fluid feeder 33 is connected to a first tube 13 through a first orifice 23 while being done to a second tube 15 through a second orifice 27. The same amount of air can be supplied to the tubes 13 and 15 to allow the supplying of air without affecting a differential pressure. Air can be blown off at tip openings 17 and 19 of the tubes 13 and 15 to prevent the clogging of the openings 17 and 19 with dust or the like. Thus, even for a fluid containing a large amount of dust, flow velocity can be measured with a differential pressure measuring device 21 continuously for a long time.