A system consisting of a moving mass point and a structure is idealized by the finite elements. A varying mass type equation of motion is obtained by considerring moving relation between displacements of the mass point and the structure. Applying the iteration method, the equation is shown to be solved by repetition of an usual constant mass procedure. Moveover, condensation of the equation enables to meet the large numbers of degrees of freedom problem. A method of mode superposition is also used to help physical understanding of calculation results, etc..