To solve an emergency problem in which both hands are full in a conventional tendency of going to school with an additional sub bag or handbag other than a satchel or a backpack, so that bags are too many and too heavy for a lower age school child, making it hard to attend school on foot because of lack of a gripping force and in which, in the occurrence of danger on the way to/from school, there is a possibility that running away or ringing a crime prevention bell takes time because the hands are full.

A bag to be mounted on a knapsack is composed of two storage bags and a first belt which connects an upper part of one storage bag and that of the other storage bag.