The flat bottle includes a cylindrical body and a bottom closing a lower opening of the body, and is formed in a flattened shape in lateral cross-section having a major axis (La) and a minor axis (Sa). A bottom wall of the bottom includes a rising circumferential wall extending upward; an annular movable wall projecting inward from the rising circumferential wall in a bottle radial direction; and a recessed circumferential wall extending upward from the movable wall. The movable wall is movable around a connected portion with the rising circumferential wall. The length of the bottom along the major axis is 1.2 to 2.0 times the length of the bottom along the minor axis. The length of the movable wall along the major axis is 0.8 to 2.5 times the length of the movable wall along the minor axis.