The flames were noticed in the building, located in the city's business district of Dalhousie, at around 8.45 am, fire brigade officials said.
Ten fire tenders were rushed to the spot and the flames were doused around 11 am, they said.
Several official documents were destroyed in the incident caused after the server room at the first floor caught fire.
The divisional officer of north Kolkata Fire Department suffered minor burn injuries, the officials said.
