PURPOSE: To improve the sealability of a top part, a bottom part, etc., and thereby prevent the lowering of filling speed by a method wherein heat seal resin layers are formed on the innermost polyester sealant layers of two bonding parts at the upper and lower ends of a blank board.

CONSTITUTION: Heat seal resin layers 20 are formed on the innermost polyester sealant layers 36 of two bonding parts 140 and 150 at the upper and lower ends of a blank board 100 to make a square post-like paper container with a gable top. In the heat seal resin layer 20, a resin good in heat sealability is used and a method of forming this layer on the polyester sealant layer 36 is by attaching a heat sealable resin-made tape or coating a heat-sealable resin. At the time of forming the container, therefore, the sealing parts of the top part, the bottom part, etc., are satisfactorily filled with the resin to prevent the leakage of liquid and a proper sealing is possible even at a relatively low temperature.