PURPOSE: To make it possible to change flow rate oscillating frequency characteristics and to make it possible to measure the flow rate highly accurately by forming an inner wall at a specified position separated by a specified distance from a restricted jetting port of a main nozzle.

CONSTITUTION: A path 3 is communicated to the restricted jetting port 2 of the main nozzle 1. The width of the path 3 gradually becomes broader toward the downstream in the direction of the flow of the fluid that is jetted from the port 2. Slant wall surfaces 3a and 3a' on both sides in the width direction are symmetrical with respect to the central flow line of the jetted fluid. A pair of returning paths 4 and 4' are formed on both sides in the width directions along the maximum width part/minimum width part of the fluid path 3. The inner wall 5, which determines the position where the branched streams flow in the returning paths 4 and 4' from the flow path 3, is provided at the down- stream in the interference region of the flow path 3. The inner wall 5 is provided at the position where an uppermost position A is separated by 13W20 times the width of the port 2 at the distance X from the port 2 and by 1.2W2.5 times the port 2 at the distance Y from the surfaces 3a and 3a'.