PURPOSE: To obtain an easily usable, high-performance, compact flow-rate detecting device, by detecting the numbers of rotations of two magnetic rotary bodies, which are rotated by a turning axial flow, by a magnetoresistance element, and operating and processing the signals.

CONSTITUTION: When a fluid flows to an outlet port 4 from an inlet port 3 of a flow-rate detecting device, the turning axial flow is obtained by fixed blades 5. Two magnetic rotary bodies 6 and 6' are deviated by 180 degrees and turned at a speed proportional to the flow rate through a flow-out preventing body 7. Two pulses generated at every one rotation are detected by a magnetoresistance element 8 and operated and processed by a control circuit 11. Thus flow rate is obtained. In this way, the easily usable, high-performance, compact flow-rate detector is obtained.