PURPOSE: To provide a simple device for measuring and indicating flow velocity by converting the kinetic energy of fluid into electric energy, and being driven with that energy.

CONSTITUTION: A windmill in a wind 2 has its rotation speed increased with an increase in air speed, i.e., the kinetic energy of the wind. This speed 3 is converted into electric energy 6 using e.g. an AC generator 5. The output voltage 7 of the generator is approximately proportional to the speed 3 and used to cause the LED 8 of an indicator to emit light. While a current flowing through the LED 8 does not exceed a threshold, the LED 8 does not emit light and the lowest voltage 7 at which the LED 8 emits light is controlled by means of a series resistance. Therefore, the minimum wind 2 required to cause the LED 8 to emit light can be controlled, and the indicator can be made to emit light at desired wind strength.