A heat sink in an electric junction box has thermal diffusion portions having one of convex and concave shapes on the surface. The structure enlarges a surface area of the heat sink compared to a configuration in which a surface has no concavity or convexity, thereby allowing fast heat dissipation from the surface of the heat sink to the exterior of the junction box. Further, the heat sink has the thermal diffusion portions on the entire plate surface. The structure enlarges the surface area of the heat sink compared to a configuration in which the thermal diffusion portions are provided to only a portion of the surface, thereby further enhancing the heat dissipation.