I think I found the way.
There is a tool called CubismViewerGems, which can convert motion files into video files.
https://github.com/DenchiSoft/CubismViewerGems/releases

- Load *.model3.json file (ex. data\others\plugin\live2d\model\Doris_ride\Doris_ride.model3.json)
- Load *.motion3.json file (ex. data\others\plugin\live2d\model\Doris_ride\motion\4_1_1_2.motion3.json)
- Select motion and click Record Animation button.
That’s all, there will be a .mov video file in the viewer directory.