Jeju fire authorities said the diners were sent to hospital at around 8 p.m. Sunday after they showed signs of carbon monoxide poisoning.
Emergency dispatchers moved them to state-run Seogwipo Medical Center and Jeju Naval Base which are equipped with hyperbaric chambers.
Seogwipo city government and fire authorities suspect the toxic gas was generated from palm tree charcoal used at the restaurant, and are investigating the restaurant owners.
Authorities said the victims are not in a serious condition.
By Kim Arin (email@example.com)