Coalescence process of metallic atom clusters is studied by newly devised multi-resolution molecular dynamics (MRMD) method which combines ordinary molecular dynamics (MD) method with rigid body dynamics. Based on the concept that molecular or atomic motion is important and effective only in a surface region in which clusters are contacting, a set of equations of motion are derived from a single Lagrangian composed of whole variables in atomic system by using assumptions of rigid body motion according to accuracy required in each region. In simulation of two equivalent clusters' collision, comparison of MRMD results with ordinary MD results is made in respect to trajectories of cluster center, energies, and angular momentums. It is found that these methods produce coincident shapes and motions of cluster. Increase of kinetic energy obtained by using MRMD method when clusters coalesce, however, is smaller than that obtained by using ordinary MD method. It is also found that proper transmission of rotational motion in clusters' system can be possible by adopting a conservation scheme of angular momentum in rigid body dynamics. As a result, contraction of calculation time is realized by the MRMD method.