Welding process is an indispensable technology in industrial fields. However, it is very difficult to control its process and quality. In order to understand the phenomena and help to search an optimum condition, many numerical models have been constructed. In this study, a 3D weld pool numerical model which can be applied various joint geometries and welding positions is constructed. Convection in molten metal and deformation of free-surface are included in our model and a simplified heat source model is used. Our model can describe different phenomena when the welding position or the joint geometry is changed. However, since a simplified heat source model is used, we have to adjust parameters to reproduce experimental results by calculation. It is important to clarify the relationships between welding parameters and heat source properties in order to assess the calculation results quantitatively.