I fixed it securely by spreading from the inside with a screw and it was able to fix firmly. Even if the road surface is bad, it will not be shaken or misaligned.
More than anything, I was deeply moved by exquisite size and exquisite wide angle. Although it is a compact size that does not disturb the appearance and does not get in the way, a sufficient field of view can be obtained for backward confirmation. It seems that I should have used it from earlier.