The detail in geometrical shape of each tooth flank of a mass-production gear is usually not the same. Even a slight deviation of tooth flank form of several micrometers has a big influence on the running vibration/noise of transmission gear box. The influence of scattering in tooth flank form on the vibrational characteristics of gears must therefore be taken into consideration at the design stage to achieve low vibration gears. In this report, the influence of typical types of scattering, i.e. in pressure angle deviation and helix angle deviation, on the vibrational excitation is clarified in terms of the relationship between vibrational excitation and actual contact ratio. Those types of scattering decrease the actual contact ratio of a gear pair from that value of the tooth flank form designed on a draft. Gears with scattering in tooth flank form can run silenter, when some less amount of tooth flank form modification than the optimum one for gears without scattering is given. The design algorithm for optimum modification of tooth flank form with scattering is proposed.