What should a Korean Muslim do if they find a difference between the qibla direction of the mosque and the qibla direction of the compass?

The ruling on deviating from the Qibla (definition) depends on the extent of the deviation. If the deviation is significant, for instance, if the Qibla is to the west but they are praying towards the south, then this invalidates the prayer, and the Qibla must be changed. However, if the deviation is slight, such as a minor deviation to the northwest or southwest within the same general direction of the Qibla, then this does not affect the prayer. The worshiper should face that direction along with them, as it is sufficient for someone far from the Kaaba to face its general direction, not its exact spot. The Prophet, peace and blessings be upon him, said: "What is between the East and the West is a Qibla." The correct direction of the Qibla can be determined by drawing a straight line from Medina to Mecca on a map.