To relax the deformation strain at the bottom of a container by injection forming a container having a bottom portion by injecting a crystalline polyolefin, cooling the container so formed for removal from a mold, then curving the bottom of the container into a concave configuration by flame treating and heating the external surface of the container, and relaxing the forming strain by cooking it down to the normal temperature.

As a raw material resin polypropylene, high-density polyethylene produced through a low pressure process so as to have a density of 0.924 or more, medium density polyethylene produced through a medium pressure process so as, to have a density of 0.930 to 0.924 and a mixture thereof. A container formed through an injection molding process or an ordinary mold is cooled for removal from the mold, and is then flame treated to the extent that the container bottom is curved into a concave configuration. In other words, the deformation strain in the container bottom is relaxed and the crystallization of the raw resin material is promoted, whereby the flame treated surface is volumetrically compressed so that the bottom portion is curved into a convex configuration. After a flame treatment like above, the container is then cooled down to the normal temperature. A crystalline polyolefin container is obtained through the processes described above.