This paper deals with how one can estimate the vibrational behavior of gears enclosed in a gear case by measuring the exterior vibration, i. e., acceleration of pedestal, through experimental investigation. In order to compare gear vibration with pedestal vibration, accelerations of all principal directions on both gears and pedestals were measured simultaneously. It has been found that the accelerations of pedestals are well-correlated to those ones of gears, especially to the rotational acceleration. That is, the transfer function between gear acceleration and pedestal acceleration is necessary for the estimation. It was proven that the transfer function obtained through the impact excitation while the gear unit endures a static torque without rotation is applicable to the dynamic estimation of gear vibration.