The fire broke out this morning when four labourers were sleeping in the factory premises. They were trapped in the blaze and burnt to death, Superintendent of Police Sunil Chowdhury said.
Other labourers who were in other part of the factory could come out safe barring one who suffered burn injuries.
All the labourers stayed in the factory premises.
Eight fire engines fought for hours to douse the flame.
The injured was admitted to hospital.