A water-soluble porphyrin having four aminophenyl groups 5, 10, 15, 20-tetrakis(4-aminophenyl)porphine (TAPP) diazotizes with nitrite ion and produces a large spectral change (ε : molar absorptivity, Δε=εsample-εblank=3.0×105M-1cm-1, 434nm). The coexistence of L-ascorbic acid (AsA) strongly interferes with this reaction. Therefore, a highly sensitive spectrophotometric determination of AsA was developed using this effect. For the determination of trace amounts of AsA, the calibration curve was linear in the concentration range of 8.0×10-8M-5.0×10-7M (14-88ppb). The detection limit (3σ) was 1.0×10-8M (1.76ppb), and the relative standard deviation was 1.86% (2.0×10-7M (35ppb) AsA, 6 determinations). This method is more than 20 times highly-sensitive than the indophenol method as standard spectrophotometry for AsA. This method has been applied to the deternation of AsA in soft drinks and drugs.