Financial Supervisory Service has started investigating SK Chairman Chey Tae-won's mistress for her alleged violation of exchange trading. 
 
According to financial insiders on Thursday, the FSS called the mistress surnamed Kim and SK's overseas arm Bergaya International, which bought Kim's apartment.
 
Kim bought a luxury apartment unit located in Seoul at 1.5 billion won in 2008 and sold it at 2.4 billion to Bergaya in 2010.
 
The FSS is set to investigate whether Kim, who is a U.S. citizen, properly reported under Foreign Exchange Transactions Act during the process.
 
The financial watchdog is set to report to relevant agencies such as the prosecution if it found any illegal acts such as tax evasion.