  We observe body-rotation behavior of pedestrians to avoid others when they
are moving in congested situations. In such cases, body orientation often
differs from walking direction, i.e., pedestrians step sideways.
  In this paper, we focused on a deviation between body orientation and walking
direction during collision avoidance by body rotation for counter-flows in
narrow corridors quantitatively. We conducted simple experiments in which two
pedestrians passed each other. We found that pedestrians rotated their bodies
when the width of the corridor was smaller than the sum of the widths of the
two pedestrians. This behavior was explained by analyzing the geometry of two
ellipses circumscribing each other in a narrow corridor. Moreover, we developed
a preliminary model and succeeded to simulate the deviation between body
orientation and walking direction during passing observed in the experiments.
  Furthermore, we performed a cross-flow experiment, which is much more complex
and realistic than the passing experiments and confirmed that body-rotation
behavior is also an indispensable factor for complex and realistic scenarios.
