To provide an auxiliary bag for a satchel which stores a lot of materials necessary for school such as gymnastic clothes, indoor shoes, and small articles in going to school or coming home, is easily attached/detached even by a child, and used as a rucksack in being detached therefrom.

An auxiliary bag for a satchel is attached to the surface of a lid plate 1 of the satchel. The auxiliary bag for the satchel is constituted of a durable material having a water proofing effect, has the width approximately the same as width of the lid plate of the satchel, has a gusset so as to store a lot of materials and a lid so as to prevent the materials from being fallen, has a fastener in the center of one side of the gusset so as to take a material from the side, and has a pocket with a lid in the front.