We tried to separate nickel and its compounds in air particulate matter into four chemical forms : water-soluble nickel, sulfide nickel, metal nickel and oxide nickel by a sequential extraction method. By testing several nickel compound reagents for solubility to various extracts, we were able to collect quantitatively each nickel chemical form from sequential extraction with EDTA solution, citric acid solution, bromine/methanol and perchloric acid/nitric acid. Each nickel extract was measured with ICP-AES or ICP-MS. The recovery rates of each nickel form were different ; the recovery rates of sulfide nickel and oxide nickel were low, whereas those of water-soluble nickel and metal nickel were high. However, the total amount of extracted nickel forms agreed with the amount of total nickel measured by the certified method in Japan for the total nickel in air particulate matter using microwave digestion. Using the sequential extraction method, we measured each nickel form in air particulate matter at several locations in Japan. The correlation between total nickel and water-soluble nickel, total nickel and oxide nickel was found through monthly analysis.