The bodies were found in a room at a Mancha Khiri district resort after the two did not check out as scheduled at 9pm.
After they did not answer a door knock, staff used a spare key to open the room and found the bodies before calling police.
The man was identified as Prasit Rupanya, 60, a retired teacher living in Tambon Kham Khaen.
The other body was identified as a 14-year old girl student who was studying in a school in Khon Kaen’s Muang district.
Pol Colonel Yutthakarn Somuang, commander of Mancha Khiri police station, said the girl’s body was found on the right arm of the man. Both had bullet wounds to their left temples, and a 9mm pistol and two empty shells were found near the bodies.
The two were properly dressed and police found no sign of fighting in the room.
Staff told police that the two arrived in a pick-up truck and checked in at 5.40pm.
Yutthakarn said police believe the teacher shot the girl before turning the gun on himself.
Yutthakarn said police learned from the man’s friends that he was divorced from his wife and had had relations with the girl for two years. The man then learned that the girl had a new boyfriend and wanted to leave the man, and he became jealous.
Police sent the gun for a test to establish whether there were fingerprints from a third person.