A method is provided for producing a bag with a mouth member by liquidtightly melt-bonding a mouth member comprising a thermoplastic resin and a bag unit comprising a relatively soft flexible film, in which uniform surface temperature of the mouth member is realized, generation of an A-shaped gap is prevented, and a bag with a mouth member excellent in the liquidtightness of the melt-bonded part between the mouth member and the bag unit is produced and also a production apparatus therefor is provided. The method comprises a preheating step of heating a tubular mouth member while rotating it around the cylinder axis thereof, and a melt-bonding step of inserting the heated mouth member into the opening part of a bag unit and pressing, and thereby melt-bonding the mouth member to the opening part of the bag unit.
