PURPOSE: To measure even a minute flow rate accurately and readily, by measuring the time period, during which a separating body such as a bubble that separates a fluid in a pipe moves over a specified distance between two points together with the fluid.

CONSTITUTION: Solenoid valves 11 and 14 are opened and 12 and 13 are closed, and a fluid 1 is made to flow clockwise in a measuring pipe part 3. A separated body 8 formed at this time moves in the pipe part 3 together with the fluid 1. Said separated body 8 is detected by photoelectric sensors 9 and 10. A flow rate is obtained from the time period, during which the separated body 8 passes the part between both sensors 9 and 10. When the separating body 8 passes the sensor 10, the solenoid valves 12 and 13 are opened and 11 and 14 are closed, and the fluid 1 is made to flow counterclockwise. Then the separating body 8 flows in the reverse direction together with the fluid 1 in the pipe part 3, and the flow rate is measured again. This operation is repeated and the flow rate can be continuously measured. Thus even the minute flow rate can be measured accurately and readily.