Technique for controlling the toughness of weld metal in structural metal materials are metallographically characterized to establish a scientific basis to produce joint materials featuring high toughness. The microstructures of the weld metal and the heat-affected zone are thoroughly investigated utilizing various methods such as optical- and electron-microscopy observation, elementary analysis, Vickers hardness test and Charpy impact test. Formation processes of the microstructures and their relation to joint performance are discussed from the material scientific viewpoint.