A tonneau cover device comprises a tonneau cover and guide rails. A front end of the tonneau cover is pivotally supported at the vehicle. The tonneau cover comprises plural board members provided in a longitudinal direction and jointly connected to each other. The board member is supported by the guide rails to move vertically. A change of a longitudinal distance between a support pin portion of the board member at the guide rail and a pivot axis portion of the board member when the board member moves upward along the guide rails is absorbed by the joint connections of the tonneau cover. Accordingly, the tonneau cover can be opened widely and the tonneau cover can allow even large baggage to be carried in or out of the baggage compartment smoothly.