Mohan Thapa, 50, a resident of Singamari under Darjeeling Sadar police station, was found guilty of raping the 15-year-old girl between Sep, 2010 and May, 2011.
Partha Pratim Chakraborty, additional sessions judge, first Court, Darjeeling, Thursday ordered Thapa to pay a fine of Rs.50,000, and in default, further undergo two years' simple imprisonment.
The victim was staying with him, while her parents used to reside separately. Thapa used to force the victim to sleep whenever his wife was away from home.
The incident came to light when the victim's teacher noticed changes in her physical appearance and on enquiry the victim said she was being continuously raped by her grandfather.
On medical examination, she was found pregnant, and delivered a girl in June, 2012.
The victim is now in Kalimpong and studying in a school run by missionaries.
The court further directed that the fine, if realised, should be paid to the victim.