Rescue operations are on and the toll could rise, said officials.
The bus was on way from Shimla to Savera Khad in Sunni area of the district when the accident occurred near Kadhar Ghat in Basantpur area, around 40 km from here.
Superintendent of Police D.W. Negi told IANS that the bus fell into a 400 m deep gorge. He said the cause of the accident is yet to be ascertained.
Locals said the accident spot has been experiencing continuous rainfall for the past many days, making the road slippery.
The injured have been taken to the Indira Gandhi Medical College and Hospital in Shimla.