The embodiment provides a tonneau cover device. The tonneau cover device includes a tonneau cover and a fixing device to engage with a lock portion to thereby hold the tonneau cover in a pulled-out-and-held state. The fixing device includes: a slider provided in a slidable manner and a hook provided in a swingable manner. When a force is exerted on an engagement protruding portion of the hook as a result of pushing in the tonneau cover toward the winding direction from the pulled-out-and-held state, and the exerted force acting on the engagement protruding portion is larger than the normally-given urging force acting on the engagement protruding portion, the hook swings such that the engagement protruding portion retracts into the case while causing the slider to be pushed in away from the sidewall, thereby being disengaged from the locking portion.