A vehicle back door comprising: a door inner panel that is formed of a fiber-reinforced resin member, that is arranged inside a vehicle interior, and that constitutes a back door inner plate; a door outer panel that is formed of a resin member that has a larger linear expansion coefficient than the fiber-reinforced resin member, the door outer panel being joined to the door inner panel, being arranged outside the door inner panel with respect to the vehicle interior, and constituting a back door outer plate; and a slack portion that is arranged in a vehicle outer surface of the door outer panel inward of a joint portion of the door outer panel joined to the door inner panel in a vehicle width direction, the slack portion including a first slack portion extended along a first direction and a second slack portion extended along a second direction; wherein the first slack portion has a convex shape or a concave shape in a cross section perpendicular to the first direction, the second slack portion has a convex shape or a concave shape in a cross section perpendicular to the second direction; characterized in that the vehicle back door further comprises an intersection portion that is arranged below a water drainage hole in a vehicle vertical direction and that constitutes a part of the slack portion, the water drainage hole penetrating the door outer panel, wherein the first slack portion and the second slack portion intersect with each other to have a peak portion at the intersection portion, the first slack portion is inclined in a planar direction of the door outer panel to extend downward in the vehicle vertical direction as the first slack portion extends outward in the vehicle width direction, and the second slack portion is inclined in the planar direction of the door outer panel to extend downward in the vehicle vertical direction as the second slack portion extends outward in the vehicle width direction. a door inner panel that is formed of a fiber-reinforced resin member, that is arranged inside a vehicle interior, and that constitutes a back door inner plate; a door outer panel that is formed of a resin member that has a larger linear expansion coefficient than the fiber-reinforced resin member, the door outer panel being joined to the door inner panel, being arranged outside the door inner panel with respect to the vehicle interior, and constituting a back door outer plate; and a slack portion that is arranged in a vehicle outer surface of the door outer panel inward of a joint portion of the door outer panel joined to the door inner panel in a vehicle width direction, the slack portion including a first slack portion extended along a first direction and a second slack portion extended along a second direction; wherein the first slack portion has a convex shape or a concave shape in a cross section perpendicular to the first direction, the second slack portion has a convex shape or a concave shape in a cross section perpendicular to the second direction; characterized in that the vehicle back door further comprises an intersection portion that is arranged below a water drainage hole in a vehicle vertical direction and that constitutes a part of the slack portion, the water drainage hole penetrating the door outer panel, wherein the first slack portion and the second slack portion intersect with each other to have a peak portion at the intersection portion, the first slack portion is inclined in a planar direction of the door outer panel to extend downward in the vehicle vertical direction as the first slack portion extends outward in the vehicle width direction, and the second slack portion is inclined in the planar direction of the door outer panel to extend downward in the vehicle vertical direction as the second slack portion extends outward in the vehicle width direction.