PURPOSE: To detect even the minute flow rate by arranging the movable ball having a plurality of groove holes in an input and output path of fluid.

CONSTITUTION: Sixteen circular grooves 6 are provided on the smoothly finished surface of the movable ball 5 which is arranged in the input and output path 3 of the fluid, with approximately equal intervals being provided. The size of said circular groove 6 is about 1/6 the diameter of the movable ball 5. The diameter of the movable ball 5 is slightly smaller than the interior diameter of the annular path 3. The specified gravity of the ball 5 is approximated to the specific gravity of the fluid. Therefore the fluid friction force of the movable ball 5 is largely increased. On the other hand, the mechanical friction of the movable ball 5 is decreased. Thus a large thrust is obtained, and even the minute flow rate of the fluid is detected.