Reinforcing structure of the resin back door

PROBLEM TO BE SOLVED: To provide a reinforcement structure of a resin back door which has effectively improved rigidity without leading to an increase in the number of parts and assembling man-hours.SOLUTION: A reinforcement structure has a back door inner panel 100 which includes a window portion 140, a back door upper portion 110 which is provided on a vehicle upper part from the window portion 140 and extends in a vehicle width direction, a back door skirt portion 130 positioned at a vehicle lower part from the window portion 140, and a pair of back door side portions 121 and 122 which are positioned on both of right and left of the window portion 140 and connect the back door upper portion 110 and the back door skirt portion 130, where the back door skirt portion 130 has an annular swelled portion 132 positioned on its outer periphery and a recessed portion 133 positioned inside of the annular swelled portion 132, the annular swelled portion 132 is welled toward the vehicle front part and is connected with the pair of back door side portions 121 and 122, and the recessed portion 133 is recessed toward the vehicle rear part.SELECTED DRAWING: Figure 1