A spectrophotometric method using bromocresol green (BCG) is proposed for determination of bicarbonate ion (HCO3-) in a small amount of environmental water samples. This method is based on variation of absorption spectrum of BCG accompanied with neutralization of an acid solution by HCO3- : H++HCO3- → H2CO3. Relation between HCO3- concentration and absorbance ratio at two wavelengths of 445 nm and 616 nm (A616⁄A445) in a concentration-defined HCl solution with BCG can be theoretically expressed by the equation derived from combining of the Lambert-Beer's law and the Henderson-Hasselbalch equation. The serious dissolution of atmospheric CO2 into pure water and standard solutions for the calibration curve can be evaded by a theoretical correction using pH and water temperature measured in laboratory, and then HCO3- from several mg⁄L to ca. 500mg⁄L in sample water of less than 0.3 mL can be rapidly determined. Comparing the new method to the conventional titration method using some environmental waters, mean analytical values obtained by this method are close to those from the titration method and the coefficient of variation of this method is shown to be about 2 times that of the titration method. An application of this new method to drip waters in a limestone cave, whose outflow rates were extremely low (several mL⁄min.), offers the possibility that this method is especially useful when sample water volumes are restricted.