PURPOSE: To provide a back door opening/closing mechanism which can make the holding force to be at maximum at full opening of a back door and become in the compact received condition at full opening and closing of the back door.

CONSTITUTION: This back door opening/closing mechanism is provided with a door side link member 6 of which one end is rotationally movably connected to a back door 1, a vehicle body side link member 8 of which the lower end is rotationally movably connected to a vehicle body 2 and the upper end is rotationally movably connected to the other end of the door side link member 6, and a damper stay 11 of which the lower end is rotationally movably connected nearly coaxially to the lower end connected part 7 of the body side link member 8 and the upper end is rotationally movably connected to the middle part in the lengthwise direction of the door side link member 6. At full opening and full closing of the back door 1, the door side link member 6, the body side link member 8, and the damper stay 11 are arranged along the nearly same straight line.