It was under a Japanese walnut tree.
I took a walk along the Tama River mid-July in the morning. When I checked the base of the usual Japanese walnut tree, I found a male stag beetle walking around energetically. It was dark, so I had a hard time taking pictures.
When I went around to the back of the Japanese walnut tree and looked at its base, there was a large longhorn beetle. A Japanese Alder Longhorn Beetle was walking around energetically. When I approached it to take a picture, it flew away. Fortunately, it landed on the ground nearby, so I was able to observe it carefully.