In this paper, first of all, the group walking behavior by four pedestrians are observed. In the observation, not only the motion data but also the decision making of each pedestrian are collected by using special device. Then, three behavioral indicators: deceleration, detour amount, and decision entropy, are defined and calculated. It has been found that these three indicators successfully quantify the ‘smoothness’ of the group walking behavior. Finally, the principal component analysis (PCA) is applied to the three dimensional indicator data. As the result, the meaning of three principal components are clearly explained. The discussion based on the PCA will be a basis for the further analysis and classification of the group walking behavior.