Golden stag beetle (likely)

Forests and woodland, especially around decaying logs; native to Australia, not Korea.

Adults consume tree sap, nectar, and fruit juices; larvae feed on decaying wood.

Males use enlarged mandibles in contests for females. Adults are mainly active in warm weather, including late summer, and are attracted to lights.

Harmless to people; larvae help decompose dead wood. The identification conflicts with the stated Korean location: this distinctive orange-and-black species is Australian and is not expected as a wild Korean insect, so an imported or captive specimen, mislocated capture, or a similar species should be considered.