A colorimetric determination of micro-amount of mercury by dithizone method in the presence of large amounts of heavy metals has been improved for the purpose of measuring trace of mercury in foods. The method is as follows: Shake the acidic sample solution (1 to 2 N H_2SO_4) with 10 ml of dithizone-chloroform solution (12 mg/l) in the first separating funnel. Transfer 5 ml of the extract to a second separating funnel containing 25 ml of 0.1 N H_2SO_4. Add 0.15 ml of 10% Kl and shake. Discard the chloroform layer as completely as possible. Rise the pH of the ageous phase to 5 with Na_2HPO_4-K_2CO_3 buffer. Add 5 ml of dithizone-carbon tetrachloride solution (6 mg/l) and shake. After discarding the first 0.5 to 1 ml, collect the clear carbon tetrachloride layer and measure the absorption at 490 mμ. It is possible to determine 5 to 10 μg of mercury in the presence of Mn 1 g, Cu 10 mg, Fe 10 mg, Zn 10 mg, Pb 10 mg, Co 100 μg, Ni 100 μg, Cd 100 μg, Bi 100 μg, Sn 100 μg and Al 100 μg by the method. Interference with silver, disregarded by other workers, has been studied and excluded by the method using Kl.