Rei and I went for a walk in the forest and came across three small longhorn beetles running around on a freshly cut log. They would not keep still, which made taking photos difficult! At first I thought they might belong to two different species, but it turned out that the red one is the female and the darker ones (big and small) are both males.
This species is a pest of cedar trees and I found an information page for it here: www.invasive.org
Japanese cedar longhorn beetle = hime-sugi-kamikiri
Location: In the forest above Yokoyama Village in Munakata (map).