The suspect set fire to his own flat, located on the fourth floor of a residential building at around 04:25 on Wednesday (19:25 GMT Tuesday), the police chief of Jinju told reporters at a press conference.
He then went down to the second floor where he knifed residents coming out of the corridor.
The victims include a 12-year-old girl, a man in his 70s, two women in their 60s and one woman in her 30s, reported Yonhap.
He was detained at the scene after another resident alerted the police. Fire-fighters extinguished the flames in his apartment some 20 minutes later, say Yonhap.
The suspect had been living alone in the apartment since December 2015, police said.