About 800,000 tones of wastes have been piled up or buried illegally at prefectural boundary area between Aomori and Iwate prefectures. Over 200 drums that contained organic solvent were buried and the solvent leaked from the drums resulting into the pollution of soils and groundwater. Dichloromethane (DCM) is one of pollutants and its concentration in groundwater was over 100 ppm. We have studied whether indigenous bacteria in the groundwater in the polluted site degraded DCM under aerobic conditions. Gas-tight vials with 10 ml of groundwater samples and 5 ml of headspace were shaken at 25°C. After 15 days of shaking, DCM concentration in a sample decreased from 108 ppm to 25 ppm. The bacteria population was subcultured using DCM as substrate without other organic matters. The bacteria population degraded DCM, and chloride concentration increased, indicating that DCM was degraded biologically. The effects of pH and TCE were investigated on the biodegradation of DCM. The experimental results indicated that bioremediation by indigenous bacteria is promising way to remedy DCM contaminated soil and groundwater at the illegal waste dump site.