Structure of door opening/closing apparatus

In a door opening/closing apparatus allowing a door opening to have as wide an area as possible, an improved structure is provided which can serve to obviate the need for including a reinforcement structure or an additional seal structure, so that the door opening/closing apparatus can be simplified in structure. An arm member 3 is coupled to a door of a vehicle, and a driving unit 2 is provided inside a vehicle body of the vehicle to move the arm member 3. The arm member 3 is arranged to be protrudable from inside of the vehicle body, through a space which the door D occupies when closed, pushing the door D open and pulling the door D shut. A seal member 8 is provided along an inner peripheral edge of a body panel PB defining a door opening S of the vehicle body, and a through opening 9 through which the arm member 3 is protrudable is formed in the seal member 3.