PURPOSE: To easily form a synthetic resin container, the cross section of a peripheral wall of which is nearly an S-shape, by a method wherein the peripheral wall of a semi-finished product under semi-cured state is introduced in a middle force and in a top force, both of which respectively have specified shapes in order to deform its peripheral wall.

CONSTITUTION: A bottomed cylindrical semi-finished product 1 equipped with a bottom wall 11 and a peripheral wall 12 is formed out of synthetic resin material. After the semi-finished product 1 is held under semi-cured state in a bottom force 4, the peripheral wall 12 of the semi-finished product 1 is introduced in a middle force 3 having an intermediate part forming surrace 33 with an upward converged shape so as to form the upward converged shape by abutting the outer surface of the peripheral wall against the intermediate part forming surface 33. at the same time, the upper end part is introduced in a top force 2, which is arranged above the intermediate part forming surface 33 and has an upper forming surrace 24 with an upward divergent shape so as to expandingly deform the upper end part of the peripheral wall by abutting the inner surface of the peripheral wall against the upper forming surface 24. Thus, a synthetic resin container, which is equipped with an upper peripheral wall 12a having an upward diverged shape and an intermediate part peripheral wall 12b having an upward converged shape, is formed. After that, the middle force 3 is expanded so as to release the peripheral wall 12 of the container from the top force 2 and the middle force 3.