This paper describes the behaviors of rotational vibrations of gears rotating under such load as the tooth deformation can be ignored. The souces of excitation of vibrations considered here are, 1. the torque variation ΔTf due to tooth friction, 2. the torque variation ΔTε due to the first derivative of transmission errors ε with time, and 3. the torque variation ΔTε due to the second derivative of ε with time. The first and the second torque variations (ΔTf, ΔTε) are obtained experimentally by measuring the torque variations of gears rotating in especially low speed. The third torque variation ΔTε is obtained by differentiate ΔTε with time. In this way, the all terms of excitation of the equation of vibration are determined, and the wave forms of vibrations are obtained by using analog computer. The wave forms obtained by above method are compared with that of experimental results, and it is found that they are closely consistent each other.The behaviors of the three components of vibration (Δθf, Δθε, Δθε) as the responses of the three excite forces (ΔTf, ΔTε, ΔTε)are as follows; 1. when the meshing frequencies of gears are in the range of lower frequencies than natural frequency of the gear system, the vibration consists mainly of two components Δθf and, ΔTε, 2. when the meshing frequencies are in the range of higher frequencies than natural frequency, the vibration consists of only, ΔTε>.