A video aired on television, reported to have been shot by a passenger, showed the moments before the landing, with a plane door open and wind rushing in as passengers sat nearby.
The transport ministry said in a statement police had taken into custody the man who had opened the door and authorities were investigating violations of aviation safety laws.
The ministry did not say why he had opened the door.
A transport ministry official told Reuters that the authorities were looking into whether Asiana Airlines had followed protocols to manage emergency exits.
The official said it was possible an emergency exit door could be opened if the plane was near the ground as the pressure inside and outside the cabin would be similar.
The plane was two or three minutes away from landing when the male passenger sitting next to an emergency exit opened a cover and pulled a lever so the door opened about 200m above the ground, an Asiana spokesperson said.
All onboard were seated with seat belts fastened because the plane was about to land, the spokesperson added.
