A method of manufacturing a bag equipped with at least one spout, including the steps of holding a spout(s) inserted in a bag mouth(s) by spout gripping members or inserting a spout(s) held by spout gripping members in a bag mouth(s), and sandwiching lightly the surface for sealing of the spout(s) with the bag mouth(s) interposed by heating plates for an extremely brief time so that sealing does not occur, and then retreating the heating plates once, and then again sandwiching the surface for sealing of the spout(s) with the heating plates, thus thermally attaching the spout(s) to the bag mouth(s). If the axial rotational orientation of the spout drifts from the bag width direction, the spout is rotated in the spout gripping members against friction force, so that the axial rotational orientation of the spout is corrected.