To eliminate such drawbacks of sport bicycles and folding small-wheel bicycles having no cargo box for reduction of weight and aesthetic reasons that there is no room to place one's personal effects, and that, though there is a way to attach a cargo box frame directly to a seat post using an adapter, a connection at only one point may not only damage a sensitive seat post material but also cause the frame to drop due to loosening of the connection and contact a rear wheel to be worn.

A frame 1 of a cargo box is supported at two points of the seat post and a saddle rail and a bag, a net, a holder, or the like is attached to the cargo box, so that the one's personal effects corresponding to each cycling purpose can be stored therein.