To provide a back door structure which is easy in opening/closing operation and sufficient in consideration of rain water.

A pair of four-link hinges 16 comprising a first link 17 in which a back door 12 is vertically divided into a back door upper 13 and a back door lower 14, and the back door is arranged substantially in the vertical direction when the back door is fully closed, a second link 19 in which one side thereof is turnably connected to the first link 17 at an upper connection point 18 at an upper end of the first link 17, a third link 21 in which one side thereof is turnably connected to the first link 17 at an intermediate connection point 20 below the upper connection point 18, and a fourth link 23 in which one side thereof is turnably connected to the first link 17 at a lower connection point 22 below the intermediate connection point 20. The other side of the second link 19 and the other side of the fourth link 23 are turnably fitted to the back door upper 13, and the other side of the third link 21 is turnably fitted to a side edge of an opening part at a rear end of the vehicle. When the back door 12 is opened, the back door lower 14 can be slipped into the lower side of the back door upper 13 by the four-link hinge 16.