Seoul's Defense Ministry said Wednesday it is investigating whether Rain broke military rules by meeting actress Kim Tae-hee while on duty.
Rain is fulfilling his compulsory army service as an entertainer for the military. He is not allowed to have private meetings while outside his Seoul base for official duties such as recording and performing.
The ministry says a brief lockup in a military jail cannot be ruled out as a penalty.
Kim's agency admits that the two have dated for a month. Rain's agency neither denies nor confirms it.
Rain joined the military in October 2011 and is scheduled to be discharged in July.