Trace metals in water samples, such as tap water, river water, and sea water, were determined by inductively coupled plasma-mass spectrometry (ICP-MS). ICP-MS has sufficient sensitivity to detect even below the concentrations of ppt level, and therefore the samples were not pretreated with a concentration column. Practical samples were diluted by lo-fold with ultrapure water and measured directly by ICP-MS. The measuring time of one sample was 2.5 min, and the data for 30 elements were obtained simultaneously. The concentrations of heavy metals, such as Cr, Mn, Co, Ni, Cu, Zn, As, Cd, Sb, and Pb, in the water samples were in the ranges of 0.05~82ng/ml. Zasu river and Sibukawa (sea water) contained various kinds of metals, and the concentration ranges were spread in a wide range; for example, Mg was 25700 ppb and In was 2 ppt. ICP-MS is found to be a useful and a powerful instrument for trace amounts of elements, and can be applied satisfactorily to the environmental water analysis.