PURPOSE: To continuously obtain the flow rate of liquid by regarding the flow rate being generated during buret switching when measurement is disabled to be the same as that immediately before switching and then interpolating the flow rate of the liquid.

CONSTITUTION: An upper quantity QU and a lower quantity QL of liquid injected into two burets 11 and 12 with the same capacity and in the same shape are detected by photoelectric sensors 21 and 23 and photoelectric sensors 22 and 24, respectively, and the detection is switched alternately and the detected values are supplied to a supply target. During this switching, a measurement disable time one of approximately 1-2 seconds is generated due to the disturbance of wave front, thus actually making it difficult to measure the quantity continuously and integrally. However, during a short switching time of two burets 11 and 12, the amount of supply can be regarded as continuous by interpolating the flow rate using the value immediately before. Namely, counting of a counter for integration starts when the switch is turned on and ends when the switch is turned off and liquid is supplied by an amount of QU-QL at each switching. The amount of continuous supply is obtained by (QU-QL) × (switching count) + (Amount of supply of buret on integration completion).