To ensure strength for partitions while making perfectness at the partitions confirmable by a method wherein the partitions dividing a sealed-up housing part of a bag into a plurality of housing sections are made of fasteners, each made of a male-and-female fitting part, provided integrally at the inside of the bag.

With one side of a mixing bag 10 put in the state of being opened, a housing section 11A is filled with stuff that is put into the section through the opening, and after that fastening members 13 and 14 are fitted together and fastening is made with a fastener 12. Then, a housing section 11B is filled with stuff 1B that is put into the section through the opening, and the opening is shut by heat sealing. On using the mixing bag 10, parts on the opposite surfaces near a position where the fastening members 13 and 14 are heat sealed are pinched with thumbs and forefingers of both hands, and fitting of the fastening members 13 and 14 are deformed and unfastened. Thereby, stuff 1A in the housing section 11A and stuff 1B in the housing section 11B are mixed together.