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