Abstract: The flow meter is free from an external fluid flow noise. The detector has measuring pipe 1, inlet 2 and outlet 3 both axially lined up with base axis X. Pipe 1 is a straight one with opposite upstream and downstream closed ends, provided in each of which ends is echo sounder 4a, 4b capable of both transmitting and receiving the ultrasound. Inlet 2 is connected with a side of the upstream end via bent 2a. Outlet 3 is connected with another side of the downstream end via another bent 3a, which another side is radially opposite to the former side part in pipe 1 so that pipe 1 is tilted at a certain angle from base axis X which is axially lined up with axes of both inlet 2 and outlet 3; and, a center of base axis X coincides with a center of a flow path of a measured fluid inside pipe 1.(See FIG.