Hi Chiharu,
Please click here to review my analysis of your serve. The lessons and drills recommended to review are:
https://members.tennisevolution.com/topics/stance-parallel-feet/
https://members.tennisevolution.com/topics/toss-x-marks-the-spot/
https://members.tennisevolution.com/topics/finish-dirty-diaper-finish/
https://members.tennisevolution.com/topics/1-2-serve-break-down/
Best,
Herman Krackow,
Head Coach