To provide a side panel reinforcement structure in which a side panel without having a door can be effectively reinforced, and which can be utilized as a trim lock means when necessary.

On a side panel 11 to compose a car body side part, a front skeleton reinforcement member 20, a back skeleton reinforcement member 21, and an intermediate skeleton reinforcement member 22 are provided in the vertical direction. These skeleton reinforcement members 20, 21, 22 substantially compose a closed cross sectional surface, and an upper end part and a lower end part of each are fixed to car body side members. A side impact bar 40 is provided covering the skeleton reinforcement members 20, 21, 22. The side impact bar 40 is disposed in the axial direction of the side panel 11, and it is fixed to the skeleton reinforcement members 20, 21, 22 by welding or the like. A trim 15 as an interior member is supported on a trim lock part 52 of the side impact bar 40 by a fixing member 51 such as a clip.