To obtain a bike bag that has a function of a wheeled bag for carrying a bicycle folded and a function of a small-article-carrying bag in which cubic volume is reduced to make the bag attachable to a bicycle.

The bike bag for a foldable bicycle includes a carrier part with wheels built in or attached from the outside and a rectangular storage part in a shape of a travelling suitcase. In the bag, it is equipped with an attachable/detachable fastener mechanism in a boundary part between the longitudinal face and a belt-like side faces of the rectangle, that the longitudinal face and the belt-like side faces in one of the bisected two regions, in an assumed cross sectional view of the rectangle, are detached by the fastener mechanism and folded into the other region, and that a folded bicycle is stored in the rectangular part when the bicycle is carried while one part of the bisected rectangle is folded and tucked into the other part and attached to the bicycle as a bag to put small articles in when the bicycle is used on a road.