A door is pivotally secured to the back of a vehicle. A door opening device comprises a connecting rod including an upper end and a lower end. The upper end of the connecting rod is pivotally mounted to the door. The door opening device further comprises a guide member having a guide slit, a rack bar which slides up and down in the guide slit, a connecting shaft that pivotally connects the lower end of the connecting rod to an upper end of the rack bar and a driving unit for opening and closing the door via the connecting rod. The lower end of the connecting rod is pivotally coupled to the upper end of the rack bar.