In this flow meter, a detector (5) is fitted slidably in a cylindrical chamber (3a) in a meter body (2), and a fluid is introduced at a predermined angle from introduction passages (17) into the interior of this detector (5), the fluid introduced into the detector (5) being discharged at a predetermined angle to the interior of the meter body (2) through discharge passages (13). A force generated by a variation in momentum of the fluid is applied to the detector (5), and this force is detected by a load measuring instrument (6) and thereby momentarily determines a mass flow rate of the fluid. Thus, a variation in momentum is generated in the fluid by the introduction passages (17) and discharge passages (13), and the force produced by the momentum variation is applied to the detector (5), the force applied to the detector (5) being detected by the load measuring instrument (6). Accordingly, the mass flow rate of a cavitation-generating fluid and a bubble-containing fluid can be measured accurately without being influenced by variations in viscosity thereof resulting from variations in temperature.