To provide an outside plate extension structure for easily suppressing distortion of the outside plate.

In a weld nut 7, an adjusting bolt 10 optionally adjusting and pressing a hollow square pipe 5 in the outward direction (arrow X) of a vehicle body 1 is fitted, and the adjusting bolt 10 is provided a thread part 10a penetrating a pillar 4 and a bracket 6 via the weld nut 7. When the adjusting bolt 10 is fastened, the thread part 10a penetrates the bracket 6 and applies a pressing force to the inside face 5b of the hollow square pipe 5 in the outward direction X of the vehicle body 1 while directly pressing and deforming a bracket interference preventing member 8. In this way, the hollow square pipe 5 is pressed in the outward direction X of the vehicle body 1, so that the outside plate interference member 9 is pressed to the outside plate 3 compressedly, and consequently, the outside plate 3 is extended outward.