A biaxially stretched, blow molded synthetic resin bottle has a bottom including a sunken bottom portion, and deforms as it draws upward in a direction of the bottle inside and includes a ring groove formed by being successively connected to an inner peripheral edge of a ground contact portion disposed at the foot of an outer peripheral wall of the bottom, a central concave portion disposed at a center of the bottom, and a flat ring portion disposed between an inner peripheral edge of the ring groove and the central concave portion, wherein the sunken bottom portion includes a plurality of short slim ribs disposed at several points of the flat ring portion.