PURPOSE: To measure precisely a flow rate of a small quantity liquid whose measurement of the flow rate is difficult by measuring a liquid level of a liquid supply tank and providing a transmitting type liquid level indicator changing a measured value to an output signal, an arithmetic device calculating the flow rate of supplied liquid and a display device indicating the flow rate.

CONSTITUTION: The supply liquid in a supply tank 1 is discharged continuously by a pump 2 through a supplying pipe 3. As the supply liquid is discharged, since the liquid level height of the tank 1 is decreased to h2 from h1 during a prescribed time Δt, the liquid level positions of h1 and h2 are measured at the prescribed times (t1, t2) by a liquid level indicator 4 with fixed time interval, the results are transduced to signals, and the signal outputs are inputted to an arithmetic device 5 connected to the indicator 4. The decreased value Δh of the liquid level is calculated at the device 5 basing on the inputted signals of h1, h2, thereby the flow rate per unit time is calculated. This flow rate calculation is carried out by multiplying an inputted sectional area of the liquid supplying tank by the value Δh, and by dividing with a prescribed time Δt required for decreasing the liquid level to h2 from h1. The calculated flow rate is displayed on a display device 6 combined with the device 5.