This paper describes adaptive motion of a biped robot with the flat soles. The robot obtains the information of reaction force from the ground by sensors at each corner of the soles. From the sensor information, environment modes are extracted. By utilizing environment mode information, the robot can detect unevenness of the environment. In some cases, the robot cannot achieve stable walking motion because of the unstable contact between the sole and the ground. In order to walk stably on the uneven ground, the robot should contact with three corners of the sole since 3-point contact is stable. Therefore, in this research, we propose a method that is applicable to the robot on the uneven ground. 3-point contact is achieved by determining the environment modes.