To provide a school bag having an auxiliary storage capable of storing a relatively large object such as a gym suit in particular and being non-obstructive when being non-used.

This school bag is provided with: a school bag body which has a body part 2 consisting of a trunk part 3 formed of a bottom plate 4 and a pair of large gussets 5 continuously provided upward at both ends of the bottom plate 4, a back plate 7 attached to the back side of the trunk part 3, and a large partition plate 6 attached to the front side of the trunk part 3; a lid 8 openably/closably attached to the body part 2; and a pair of shoulder belts 9 attached to the back side of the body part 2: and the auxiliary storage 21 detachably mounted on the school bag body. The auxiliary storage 21 has a pair of base parts 22 mounted on the pair of large gussets 5 respectively, and a storage body 23 having leg parts 24 downward movably inserted into the respective insides of the base parts 22, and a storage part 25 communicating between the pair of leg parts 24 and each other.