PURPOSE: To prevent a folded part from whitening by a method wherein only parts which are to be fold lines when a hardened synthetic resin hollow body of a container body is folded are softened.

CONSTITUTION: In a container consisting of a hollow body 11 made of hardened synthetic resin and lids 12, 13 attached to the upper and lower ends of the body 11 freely removable, a part 7' to be a fold line when the hollow body 11 is folded is softened. To soften it, the part 7' which is to be partially softened. To soften it, the part 7' which is to be partially softened in a hardened synthetic resin sheet 7 is heated, the heated part is drawn, and then it is subjected to vibration of 100 to 104 Hz order, cooling and recovery approximately simultaneously. Polystyrene, ABS, etc. can be used as the hardened synthetic resin. Thus a whole container can be folded irrespective of the shape of the hollow body and is free from whitening on the fold lines to be equal in appearance to other parts of the hollows body so that appearance is not spoilt.