An image sensor includes a housing which receives heat of a circuit board, and a light source which is supported in the housing and radiates light toward a document. The light source is an EL light-emitting element including a light emission section which emits light by organic electroluminescence. The EL light-emitting element extends in a line shape along a scanning direction. A thermal diffusion layer is interposed between the EL light-emitting element and the housing.