Light elements in metallic metals play important roles in determining the properties of materials. Hydrogen is already present in, or easily penetrates, many metals. The carbon content affects the ductility of iron. Recently, as the refining process of iron ore has become improved, the importance of determining the impurities in iron has become greater. However, although the impurities of iron materials are controlled in the refining process, their contents and distributions are changed during the manufacturing process. In order to monitor the variation in the composition of light elements in sample surfaces during a welding process, TOF-ERDA using high-energy heavy ions has been applied to determining the composition of light elements in stainless-steel (SUS304) samples before and after welding. Three stainless steel sheets were cut into three pieces, respectively, and two pieces of each sheet were welded. An argon-welding method using a welding rod (SUS304), an argon-welding method not using a welding rod and an arc-welding method using a welding rod were used to prepare samples. Six samples, welded and non-welded using three different welding methods, were measured. 40Ar ions accelerated to 41.5 MeV were used as a probe. Hydrogen, carbon, oxygen and fluorine were measured. It was found that the oxygen distributions near to the surface of the welded samples decreased compared with those of the non-welded samples for all welding methods. However, no explicit variations in the carbon distributions were observed.