The police write on their website that the restaurant was empty and that no one was injured.
However, glass panes were smashed in the premises.
The alarm came at 4 o'clock on the night of Monday.
A preliminary investigation has been launched and the police have classified the explosion as public destruction.
- We believe that something has been thrown into the room which then exploded.
All the windows in the restaurant have been smashed, but there does not seem to be any major devastation, says Pontus Hjelm, commander of police region Mitt, to TT.
The National Bomb Squad is on its way to the scene to conduct a crime scene investigation.
The site is cordoned off.
No people have been detained in the case, the police say.