In order to provide a roof box system (100) for a motor vehicle (11) which is easy to handle and which can be used in many different ways and which features an aerodynamically advantageous shape, it is proposed that the roof box system (100) shall include a roof (10) of a motor vehicle (11) which includes at least one ceiling element (13) which is installed on at least one roof base element (12) and an insertion element (18) with at least one outer wall. Pursuant to the invention, the at least one ceiling element (13) can be moved from an initial state to a receiving state so that an interspace (15) suitable for receiving the insertion element (18) is formed, and, in the receiving state, the insertion element (18) can be arranged in the interspace (15) so that the at least one outer wall of the insertion element (18) forms an outer surface (26, 27) of the roof box system (100).