To provide a reliable bag where the absolute sizes of the bag and a goods housing part are instantly changed according to the quantity of the goods.

The bag is provided with an expansion/contraction part 1 at its trunk part to vary an internal capacity. Even when the quantity of goods which increases while traveling such as souvenir cannot be estimated, the capacity and size of the goods housing part in addition to the size of the bag can be varied according to the quantity of the goods at an appropreate time such as departure and during the travel by expanding and contracting the trunk part. To the expansion/contraction part 1, various kinds of expansion/contraction means of so-called telescope structure for adjusting the relative movement of a plurality of cylinders by operation of press buttons provided with a leaf spring etc., sliding structure for relative movement along a long hole by providing the long hole for engaging the pin of one cylinder to the other cylinder in a slidable manner, or connection of the plurality of cylinders through bellows, e.g. can be applied.