The Onyasai restaurant in Fukushima was undergoing repairs, according to its owner, casual dining group Colowide Co , which operates a chain of the restaurants.
The company issued a statement apologizing to the victims and said it was working with authorities to determine the cause.
Media showed footage of the restaurant reduced to a shell, with windows shattered in nearby buildings and pieces of metal and glass scattered around the block.Some Twitter users in the area said they heard a loud bang at the time of the explosion and could smell gas.