To reduce labor and time for the attaching/detaching work, to simplify and shorten the exchanging work, and to enable anybody to surely set a pair of trousers into a factory shipment state regardless of the skill of a worker.

A setting plate 4 vertically provided is placed on the front side of a frame 2 of this trousers finishing machine 1 which finishes a pair of trousers 5 under a suspended state, and this expansion bag for the trousers finishing machine is expanded with hot air, and is fitted on the trousers 5 from the bottom side into an externally fitted state to tension-finish the waist section 5a of the trousers 5. For the expansion bag for the trousers finishing machine, bag-shaped sections 8 for each passing a string is formed on an opening peripheral edge section 6 on the upper side, and an internal peripheral section 7 around the setting plate 4 corresponding to the opened peripheral edge of the setting plate 4. The string 11 is passed through the bag-shaped section 8. Annular hooking sections 11a hooked on protrusions 12 which are properly provided at attaching locations are formed on both end sections of the string 11 which projects from the open ends of the bag-shaped section 8.