One of the ideal methods to stabilize passive dynamic walk is to reset the angle and the velocity of a swing leg to their initial values every time after a new gait phase begins. To achieve this, it was proposed to mechanically limit the range of the rotation of a hip joint. This drastically contributes to realize stable walk. But a hip joint limitation does not always ensure the same angle for the beginning of a swing leg, especially when an entire robot rapidly turns over. For an alternative approach, we propose a straight forward method where the angle of the starting swing leg is set to its initial value by constraining its angle at the bottom of its sole. We show that simply extending a sole shape toward a toe direction gives this effect. The proposed shape was applied to a 3D stick leg passive walker and achieved a long time non stopping walk. Computer simulations of a simplified stick walker model with this sole constraint shows that the mechanism widens stability of walk over a wide range of walking conditions.