A studless tire comprises a tread provided with a plurality of blocks and at least one rib provided with first sipes and second sipes each extending across the rib. The first sipes and second sipes are alternately arranged in the longitudinal direction of the rib at circumferential pitches Pc in a range of from 0.028 to 0.04 times a ground contacting width of the tread. Each of the first sipes and second sipes comprises a deep central part, a shallow part on one side thereof and a mid-depth part on the other side thereof, wherein the deep central part has a depth in a range of from 0.70 to 0.80 times a depth of a main longitudinal groove defining the rib, the mid-depth part has a depth being in a range of from 0.40 to 0.60 times the depth of the main longitudinal groove, and the shallow part has a depth in a range of from 0.04 to 0.35 times the depth of the main longitudinal groove. The shallow part and mid-depth part of each first sipe are positionally reverse to those of the second sipes so that, on each side of the rib, the shallow parts alternate with the mid-depth parts.