Insect species identification
It is a moth and all moths and butterflies belong to the order of Lepidoptera. Based on the shape of the forewing and the orange color of the hindwings I think this moth belongs to the genus Catocala https://en.m.wikipedia.org/wiki/Catocala, see also here for some pictures.

EDIT:
theforestecologist did a excellent suggestion that this might be Catocala streckeri based on the wing pattern. This species does occur in Korea, see this website for a distribution map. So it may indeed be Catocala streckeri. However, Asia is very rich in Catacola species, and I know that this family often has species looking very alike, so I can't say with 100% certainty.

Most, but not all, moths are nocturnal. Some Catocala species are known to fly on sunny afternoons (Waring&amp;Townsend 2006).