A welding apparatus is provided with a lamp unit having a light source such as a halogen lamp and a multiple reflection surface. The light source applies incoherent electromagnetic waves that belong to a visible region and a near-infrared region in which a wavelength is 500 nm (nanometer) or more. The multiple reflection surface reflects and collects the light from the light source in the shape of a desired dot or line. The housing and the transparent cover are superimposed, and the light from the light source is collected by the multiple-reflection surface on the portion where the housing is superimposed on the transparent cover.