To form an easily opening groove part always having a proper depth to the surface of the expanded sheet made of a resin of a container, which is equipped with a flow channel between two bonded resin sheets by expanding at least one resin sheet from the surface of the resin sheet, without stopping the feed of the container.

In the method for forming the groove part for opening the container, the surfaces of the resin sheets 1a and 1b are irradiated with a laser L for forming the groove part and the forming range of the groove part along the irradiation direction is larger than the expanded quantity of the resin sheets 1a and 1b in the forming region of the flow channel 2.