PURPOSE: To make a precise mechanical working of a case main body unnecessary by providing a cylinder body having a bottom and fitting to a surrounding wall surface of a measuring chamber and by providing all of the fitting parts of a nozzle and a pivot axis to the cylinder body having a bottom.

CONSTITUTION: The measuring chamber 33 having a cylindrical circumference wall surface 33a is formed between an inflow hole 31 and an outflow hole 32 formed at the case main body 30, and the cylinder body 34 having the bottom and molded with synthetic resin is fitted to the circumference wall surface removably. A wing wheel 41 is supported turnably with a pivot shaft body 43 fitted to a center boss 42a of a cylinder body bottom 42 in the body 34, and an inflow nozzle 36 and an outflow nozzle 37 are formed at a side wall part 35 so that they are placed along a tangential direction of rotation of the wheel 41. Since especially precise working is unnecessary to the holes 31 and 32 of the body 30 and they can be formed to simple linear form, the forming of the body 30 is especially simple.