A vehicle door coupling device for mechanically coupling together a first member having a first hole inside which an engaging protrusion is provided and a second member having a second hole at a position corresponding to the first hole, the vehicle door coupling device comprising: a shaft shaped to be fitted into the first hole of the first member and the second hole of the second member and rotatably pass therethrough, the shaft extending in a vertical direction; a stopper provided on a side of the shaft and configured to support the first member; a crossbar provided on another side of the shaft and configured such that when the shaft is rotated to a final coupling position, the crossbar fixedly couples the second member to the first member while supporting the second member; and a position maintaining structure provided on the shaft, the position maintaining structure being configured to allow the shaft to be temporarily coupled to the first member in a temporary coupling position in a state in which the shaft is vertically movable without being separated from the first member and being configured to maintain a state in which a rear side of the crossbar partially protrudes from the second member in the temporary coupling position, characterized in that the position maintaining structure comprises: a vertical groove portion which is provided in an outer side of the shaft, extends in the vertical direction toward the side of the shaft from the other side of the shaft, and is configured to receive the engaging protrusion therein and guide the engaging protrusion; and a horizontal groove portion which extends in a horizontal direction from an end of the vertical groove portion and is configured to allow rotation of the shaft, wherein a position maintaining protrusion is provided in the vertical groove portion to prevent the engaging protrusion of the first hole from entering the horizontal groove portion in the temporary coupling position, wherein a separation-preventing protrusion is provided on an entrance of the vertical groove portion to prevent separation of the engaging protrusion inserted into the vertical groove portion, and the engaging protrusion allows the shaft to move upward or downward in the temporary coupling position while the engaging protrusion vertically moves between the separation-preventing protrusion and the position maintaining protrusion. a shaft shaped to be fitted into the first hole of the first member and the second hole of the second member and rotatably pass therethrough, the shaft extending in a vertical direction; a stopper provided on a side of the shaft and configured to support the first member; a crossbar provided on another side of the shaft and configured such that when the shaft is rotated to a final coupling position, the crossbar fixedly couples the second member to the first member while supporting the second member; and a position maintaining structure provided on the shaft, the position maintaining structure being configured to allow the shaft to be temporarily coupled to the first member in a temporary coupling position in a state in which the shaft is vertically movable without being separated from the first member and being configured to maintain a state in which a rear side of the crossbar partially protrudes from the second member in the temporary coupling position, characterized in that the position maintaining structure comprises: a vertical groove portion which is provided in an outer side of the shaft, extends in the vertical direction toward the side of the shaft from the other side of the shaft, and is configured to receive the engaging protrusion therein and guide the engaging protrusion; and a horizontal groove portion which extends in a horizontal direction from an end of the vertical groove portion and is configured to allow rotation of the shaft, wherein a position maintaining protrusion is provided in the vertical groove portion to prevent the engaging protrusion of the first hole from entering the horizontal groove portion in the temporary coupling position, wherein a separation-preventing protrusion is provided on an entrance of the vertical groove portion to prevent separation of the engaging protrusion inserted into the vertical groove portion, and the engaging protrusion allows the shaft to move upward or downward in the temporary coupling position while the engaging protrusion vertically moves between the separation-preventing protrusion and the position maintaining protrusion. a vertical groove portion which is provided in an outer side of the shaft, extends in the vertical direction toward the side of the shaft from the other side of the shaft, and is configured to receive the engaging protrusion therein and guide the engaging protrusion; and a horizontal groove portion which extends in a horizontal direction from an end of the vertical groove portion and is configured to allow rotation of the shaft, wherein a position maintaining protrusion is provided in the vertical groove portion to prevent the engaging protrusion of the first hole from entering the horizontal groove portion in the temporary coupling position, wherein a separation-preventing protrusion is provided on an entrance of the vertical groove portion to prevent separation of the engaging protrusion inserted into the vertical groove portion, and the engaging protrusion allows the shaft to move upward or downward in the temporary coupling position while the engaging protrusion vertically moves between the separation-preventing protrusion and the position maintaining protrusion.