The North Chuncheong Provincial fire department said it took 10 minutes to put out the fire, but it caused about 1.8 million won ($1,604) damage.
Almost 3.3 square meters were burnt and 20 residents evacuated from the building.
"I was outside when my girlfriend called me and told me that she would burn my house down," the man, who is also 22, was quoted as telling police.
Police said the woman knew the password to the apartment and started the fire after a fight, which ended in the boyfriend telling her he wanted to end the relationship.