A tonneau cover device of a vehicle equipped with a rear opening portion formed at a rear portion of a vehicle body, a back door to open and close the rear opening portion, and a rear baggage compartment formed in front of the rear opening portion, comprises a tonneau board provided so as to extend substantially horizontally over the rear baggage compartment, the tonneau board comprising plural split boards which are split in a vehicle longitudinal direction, longitudinally-adjacent split boards being coupled to each other via a hinge portion extending in a vehicle width direction, and first and second connecting members as a folding mechanism pulling upwardly the split boards when the back door is opened so that the split boards bend around the hinge portion.