At the step of preheating the container, in addition to heating of the whole container, a neck portion (1a) of the container may be heated separately. Alternatively on the step of preheating the container, while whole container is heated by a hot air blast which is supplied in the container, neck portion (1a) of the container may be heated separately. Since it is difficult to increase the temperature of the neck portion as compared with other portions, by heating the neck portion separately, the effect of the preheating is performed more effectively. It is desirable that the hot air blast is supplied from the nozzle inserted in the container. For example by the method of spraying hot air blast from the outside of the portions, heating of the neck portions can be performed.