To easily manufacture a PET bottle having its vertical width (a height) substantially reduced as compared with that of a bottle body filled with a content, in manufacturing of the main body of the PET bottle.

This manufacturing method comprises the steps of molding and forming the main body of a container where either a part of the main body or an entire main body in a vertical direction is made into a bellows shape in a horizontal direction, compressing the molded main body of the container in a vertical direction, applying the molding process for keeping the compressed state under a normal temperature and extending the main body of the container from the compressed state to one where it becomes substantially the same height as its original height (volume).