The truck appeared to reverse for 28m before accelerating towards the outdoor market, the fire official said.
The driver had blamed a sudden surge of acceleration for the incident, he said.
The driver was not intoxicated when tested for alcohol, a police official said at the briefing.
Authorities would hand the vehicle over to investigators to try to determine the cause of the crash, the official said.
CCTV footage aired on Korean television showed shoppers going about their business at an outdoor market before a blue truck accelerated along a passageway lined by stalls and crashed into one of the stores.
Police had arrested the driver, who was in his 60s, and he was being questioned, Yonhap said.
