This preform (10) is provided with a mouth (11), a body (20) with an inside diameter of D1, and a base (30). The base (30) has, in terms of cross-sectional shape, the following: an outside surface (31) that has an outside-surface middle section (35) with a radius of R1 and an outside-surface edge section (36) with a radius of R2; and an inside surface (32) that has an inside-surface middle section (37) with a radius of R3 and an inside-surface edge section (38) with a radius of R4. Said diameter and radii satisfy the relations (a) R2 < R1, (b) D1/2 < R3 < R1, and (c) R4 < R3. Giving the base (30) of the preform a flattened shape makes it possible to increase the thickness of the tip of the base of the plastic bottle that results after blow molding, preventing problems in which the tip of the base of the plastic bottle becomes crushed during shipping and handling despite being under pressure.