PURPOSE: To simply and securely stabilize a bag mouth and enable to preserve contents through closing a bag again even after opening the bag by forming the bag mouth in a planar form and joining it, and folding it in three stages so that a specific condition is satisfied.

CONSTITUTION: Finishing a pack of a bag S through closing the bag mouth IS is started by deforming the wall of the bag IS in each stage A-E. In stage F a planar part arranged in a plane containing the advancing direction of a conveyor conveying the bag S is formed. In stage G the planar part is joined by a tool 10. In stage H the joining part SA is folded double along a plurality of folding lines 12, 14 by a plurality of tools 16, 18 and a plate piece 20. In stage IS the first folded part 12 is straightened by a tool 28, and the third folding 22 is carried out by a plurality of tools 24, 26 after arranging the second folding part 14 in a symmetrical plane. In stage L the bag mouth IS is folded downward by a tool 30.