The man was trying to get off the train during the morning rush hour while thescreen door was closing at Gimpo airport subway station in Seoul.
The driver started the train without seeing him trapped there and the 36-year-old was crushed to death, Seoul Metropolitan Rapid Transit Corp. said in a statement.
"We're deeply sorry for the incident," Na Yeol, acting chief of the company, told a press conference, promising a thorough investigation.
Screen doors have been installed at all subway stations in Seoul, partly to prevent people from committing suicide.
But the doors sometimes cause casualties by malfunctioning or trapping people.
In May a junior employee was killed by a subway train while fixing a screendoor on a platform in Seoul.
