We have newly developed the interatomic potential of Si, Ge or Ge, Sn mixed systems to reproduce the lattice constant, phonon frequency, and phonon dispersion relations in the bulk pure group IV crystal and group IV alloys by molecular dynamics (MD) simulation. The phonon dispersion relation is derived from the dynamical structure factor which is calculated by the space-time Fourier transform of atomic trajectories in MD simulation. The newly designed potential parameter set reproduces the experimental data of lattice constant and phonon frequency in Si, Ge, Sn, and SiGe. Furthermore, the Sn concentration dependence of the phonon frequency, which are not yet clarified, is calculated with three type assumptions of lattice constant in GeSn alloy. This work enables us to predict the elastic and phonon related properties of bulk group IV alloys.