The blaze was caused by an explosion of flammable material being used in the installation of insulation in a basement, South Korean media reported.
All the victims are believed to be construction workers.
There are fears the death toll could rise as 78 people were working in the building and not all are accounted for.
South Korea's Yonhap news agency says at least 10 people were injured in the fire at Icheon, 80km (50 miles) south of the capital, Seoul.
The blaze spread quickly, severely hampering evacuation efforts.
It began at about 13:30 local time (04:30 GMT).
Prime Minister Chung Sye-kyun ordered the mobilisation of "all necessary personnel and resources" to tackle the emergency.
The fire was put out at about 18:40 local time.
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