The present invention is intended to provide a studless tire capable of enhancing straight-running stability with a low tread rigidity.In a section of a studless tire 10 passing a central axis line under a condition where the tire is mounted on a predetermined rim 11 and a predetermined inner pressure is charged therein, assuming that a distance A is measured from a position where a bladder ring is divided to a position where a maximum tire width is obtained along the tire radial direction and a distance B is measured from the position where the bladder ring is divided to a position where a tread ring is divide along the tire radial direction, a side-shape coefficient of the tire defined as a ratio A/B in a state where the tire is installed on a vehicle is between 0.52 and 0.55 in a widthwise half portion disposed at a widthwise outer side of the vehicle and between 0.45 and 0.50 in a widthwise half portion disposed at a widthwise inner side of the vehicle, or between 0.5 and 0.55 in a widthwise half portion disposed at the vehicle installation outer side and between 0.45 and 0.48 in a widthwise half portion disposed at the vehicle installation inner side.