In this paper the characteristics of avoidance behavior against an obstacle under free walking conditions are discussed, based on the premise that the pedestrians generally change their routes to avoid an obstacle. Here the following model is assumed. The distance (y) from pedestrians to an obstacle at the beginning of avoidance behavior is determined by the sum of three distance factors : distance to keep Personal Space (B_f), distance caused by the pedestrian's foward movement (T_p), and distance exerted by the forecasted movement of an obstacle (T_m), as follows y=B_f+T_p+T_m In order to determine the distances of each factor quantitatively, y was measured under the condition avoiding a stationary obstacle (an object or a human being) on a sidewalk. The behavior model results agree with empirical facts, as follows 1) The size of B_f is about 1.8m. 2) The size of T_p is about 3.4m. 3) T_m is determined by the characteristics of the obstacle. If the obstacle is a human being, the size of T_m is determined by the direction of the human being.