PURPOSE: To simplify constitution, to obtain long-period stability and to improve reliability by charging a battery through a power generating runner for the electronic detection and display of the measured amount of water which corresponds to the rotational frequency of a measuring runner.

CONSTITUTION: The rotations of a measuring runner 11 are detected through a magnet 10, a magnetic resistance element 17, etc., and the measured auantity of water is converted by an LSI16 into an electric signal, which is displayed on a liquid crystal display aprt 14 and also transmitted to an external reader. The electric power required for the electronic detection of the measured amount of water, the conversion to theelectric signal, the display, etc., is supplied to a chargeable dry battery 15. a power generator 24, on the other hand, is driven through a power generating runner 22 to charge the battery 15. Therefore, the electric life is prolonged, the electronic simple constitution does not perform display and drive directly by the runners, and the stability and reliability of the measuring device are improve for a long period.