To develop a practical method for getting an insight about the influence of factors such as gear dimensions gear manufacturing and alignment errors on the running performance of gears, a new simplified method to analyze gear vibration was introduced in the previous report. It is a method to reduce a forced vibrational system with parametric exciting term. In this report, the vibration of a pair of involute spur gears is dealt with, and the nature and the accuracy of the solution of this approximation method is discussed for cases where the involute spur gears have several kinds of tooth form errors and alignment errors. The solution of the approximation method and that of the original exact method were compared with measured with measured results of teh vibration level and of the frequency construction of gear vibration, and the scope of application of this simple approximation method is shown.