The analytical method for the determination of phosphorus in ultrapurified water was developed. Ultrapurified water was evaporated to concentrate phosphorus and the final sample volume for analysis was 10ml. In 0.55moll - 1 HCl, orthophosphate forms molybdophosphate, and then the molybdophosphate forms ion associate with Malachite Green (MG), which can be collected on a tiny membrane filter (diameter: 5mm, and effective filtering diameter: 2mm). After the ion associate on the membrane filter is dissolved together with the membrane filter in 1ml of methyl cellosolve (MC), the absorbance of MC solution is measured at 627nm by a flow injection-spectrophotometric detection technique. When 10ml of the sample solution was used for the procedures and absorbance measurement, the calibration graph is linear up to about 500ngl - 1 of phosphorus and the detection limit was 8ngl - 1 (S/N=3). For the determination of phosphorus in an ultrapurified water, 10-40ml of sample solutions were transferred into poly(tetrafluoroethylene) (PTFE) beaker and evaporated to 5ml or to dryness. To them, 0.003moll - 1 HCl was added to get 10ml of final solution, which was used as sample. Phosphate is determined by comparing the slope of the varied sample volume after evaporation/concentration with a slope of the standard calibration graph (a slope comparison method: SCM). The SCM enables to evaluate the concentration of phosphate in ultrapurified waters more sensitively and accurately.