To determine micro-amounts of proteins with low molecular diffusivity in biological materials, an FIA type capillary stream sensor (CSS) was developed by the dye association method using Bromocresol Green with high sensitivity and little interference. The CSS device was based on Korenaga-Stewart's S4-FIA system which was different from the traditional FIA systems because of the use of very slow flow rates for reagent and carrier streams. A Furue Science, Model J-P-VW, double-syringe pump was used, and the optimal conditions were examined with bovine serum albumin (BSA) as a standard at a slow flow rate region. As the result, less than 0.05 ml/min of flow rate was acceptable in case of using small-bore and short reaction tube (0.5 mm bore and 200 mm length). The method had analysis rate of 78 samples/h with relatively long residence time, but straight calibration graph was obtained for 8 μl of injection volume accompanied with high reliability and reproducibility. The detection limit and RSD were 0.3 μg and 0.2% for BSA, respectively. This research gave a useful CSS device technology at a viewpoint of analytical instrumentation hydrodynamics for small consumption of valuable reagent solutions and biological material samples.