There was no sign of a fire and investigators at the site believe a gas leak may have caused the blast in Koriyama, fire department official Hiroki Ogawa said.
The wounded have been taken to hospitals, with Mr Ogawa saying two of them were seriously injured.
The area has been closed off and neighbours have been evacuated.
National broadcaster NHK aired footage showing only a skeleton left of a building, the steel structure gnawed and pieces of wall and glass scattered around.
Several employees and customers at a nearby bank were injured, NHK said.
The explosion occurred in a crowded business and commercial district close to the city’s main train station, where hospitals, schools and the city hall are also located.
An employee at nearby Kuwano Kyoritsu Hospital told NHK: “I heard a ‘boom!’ and felt the ground shaking, so I thought it was an earthquake, but the shaking didn’t last very long so I realized it wasn’t.”
Some windows of the hospital were shattered but no-one inside was injured.
