To provide a method for the simple and quantitative determination of the degree of contaminant decomposition activity of fungus derived from mushroom bed.

A plastic cup is charged with 10 g of a sample produced by mixing a contaminated soil containing a contaminant with a mushroom bed containing fungus capable of decomposing the contaminant and the cup is covered with a lid and placed in a thermostatic chamber maintained to 25°C. The sample is thoroughly stirred for 1 day, 2 days, 4 days and 7 days after mixing to obtain a prepared sample. Separately, the scale of a recorder connected to an oxygen electrode for the determination of oxygen absorption rate is confirmed beforehand. The reaction vessel of the oxygen electrode is charged with 2 ml of a buffer solution and saturated with air by stirring with a magnetic stirrer. The prepared sample is added to the buffer solution in an amount of 50-100 mg and 0.6 V potential is applied to both electrodes of the oxygen electrode. The electrolytic current is read out from the chart of a recorder and the oxygen absorption rate is calculated from the value. The activity of the fungus is determined from the oxygen absorption rate.