The purpose of this study is to clarify the relationship between rotational angle and meshing vibration of a plastic gear mating with a metal gear. A newly developed data acquisition system attached to a gear operation test rig enables us to measure the meshing vibration and to capture the side-view images of the gear mating part at synchronized timing. The gear meshing vibration was acquired by an acceleration sensor installed on the bearing housing of the rotating shaft of the plastic gear. A Gaussian filter eliminated the short-wavelength components in the meshing vibration. The filtered data was cut out at every pitch rotation by a zero-crossing method. After that, a time-synchronous averaging method was performed for each rotation of the plastic gear. As a result of data analysis and checking the side-view images of the captured gear mating part, it was found that a healthy plastic gear had significant amplitude modulation in the averaged meshing vibration. The amplitude increased when the teeth near the two pinholes for transmitting torque meshed, and decreased when the 90-degree phase advanced. In addition, the large magnitude of the modulation decreased with the growth of the tooth root cracks.