TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: In (M1), the oxygen demand in soil is computed from reduction speed of the residual oxygen concentration and the soil air volume of contaminated soil. A thermometer (7) is provided into contaminated soil and the soil temperature is measured with thermometer and (M1) computes air permeability to the contaminated soil based on the oxygen demand and soil temperature. (M1) measures time-dependent concentration change of the residual contaminant in the contaminated soil by degradation of aerobic microbe and residual oxygen, defines degradation reaction formula by the aerobic microbe of the contaminated in contaminated soil in the (M1), and detects the residual contaminant concentration soil from reduction speed of the residual oxygen concentration, and degradation reaction formula. Controlling the gas flowing time to contaminated soil, or a stop in (M1) based on the residual contaminant concentration, (M1) measures the reduction speed oxygen concentration at the time of no ventilation in (M1). The increase speed of oxygen concentration in soil is measured by oximeter, (M1) computes air permeability to the contaminated soil based on the measured value of the increase speed. (M1) detects the reduction of soil space more air permeable apparent. (M1) is based on air permeable apparent and controls the air permeability to the contaminated soil. The increase oxygen concentration is measured at the time of initial stage gas flowing and resumption of gas flowing and detects the apparent air permeability at the time of the resumption of gas flowing by the comparison of the increase speed in the oxygen concentration at the time of both gas flowing. The oximeter and/or thermometer at about 2 or more of contaminated soil and controls the gas-passage to contaminated soil for each location. The contaminated soil is in the contaminated ground. A ventilation well and the a measurement well are drilled all over the ground, an oximeter and/or thermometer are inserted in the measurement well, the ventilator is connected to the said ventilation well. Preferred Organism purifier: (I) computes the oxygen demand in soil from reduction speed of the residual oxygen concentration and the soil air volume of contaminated soil and controls air permeability of the ventilator 1 controls apparatus in (I) based on the calculation value of the oxygen demand. (I) provides thermometer which measured soil temperature and computes air permeability of the ventilator based on the oxygen demand and soil temperature with control apparatus. In (I), the degradation reaction formula by the aerobic microbe of the contaminant in contaminated soil is stored in control apparatus, (I) computes the decomposition rate of the contaminant in soil from reduction speed of residual oxygen concentration and the degradation formula with control apparatus and controls the gas flowing time of the ventilator or a stop based on the calculation value of the decomposition rate. The measured value of time-dependent concentration change of the residual contaminant in the contaminated soil is stored in control apparatus. (I) controls gas flowing time of the ventilator or a stop based on residual contaminated concentration which detected the residual contaminant concentration with the controls apparatus from the calculation value of decomposition rate of the residual contaminant and the measured value of time-dependent concentration range and was detected. (I) measured the detection speed of the residual oxygen concentration with the control apparatus in the purifier at the time of no gas passage. (I) measures increase speed of oxygen concentration at the time of gas flowing by the oximeter and control apparatus in the purifier based on the measured value of the increase speed. The increase speed of the oxygen concentration is measured with the control apparatus at the time of initial-stage gas flowing and resumption of gas flowing. (I) detects the apparent air permeability at time of resumption of gas flowing by the comparison of the increase speed in oxygen concentration at the time of both gas EXAMPLE - No relevant example is given.