A precolumn reaction chamber was integrated into a polyester microfluidic device with a miniaturized detection system. The reaction chamber was designed to be a zigzag channel, 70 μ m in width, 8 mm in length, followed by a wider straight channel, 150 μ m in width, 2 mm in length. The detection system is composed of an embedded light-emitting diode (LED), an integrated optical fiber, and a photomultiplier tube (PMT). A success in amino acid analysis using the integrated microchemical analysis device proved that the precolumn reaction chamber was compatible with the integrated detection system. Three kinds of amino acids, arginine, glycine, and phenylalanine, mixed and reacted with 7-fluoro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-F) in the precolumn reaction chamber to produce fluorescent products, were separated by micellar eletrokinetic chromatography (MEKC) and detected by LED-excited fluorescence. The detection limits for arginine, glycine, and phenylalanine were 1, 1, and 0.5 mM, respectively, which can be improved by further optimizations of the reaction system and detection system.