Sujoy Ghosh's thriller Kahaani stars Vidya Balan and Parambrata Chatterjee in lead roles.
"I am very much happy winning the award. It is a huge honour. I am still trying to absorb that we have won the award. The feeling is yet to sink in," Sujoy told PTI.
"Now I feel there is more responsibility on me as a director. Somebody is putting trust and responsibility on you and I will continue to give my best and work hard," he said.
Vidya Balan starrer Kahaani has swept several awards earlier across various categories.
"I had not expected that we could get national award for the film. As this time the competition was quite tough...there were lot of other good films made last year. We did not hope to win it," Sujoy said.
In the film, Vidya played the role of a a seven-month pregnant woman who came to Kolkata from London in search of her missing husband. With nothing to rely on except fragments from her memories about him, it is a story of a lone woman's relentless struggle in an unknown city.
Immediately after the resounding success of Kahaani, word spread around that Sujoy was mulling making a sequel to the film.
"Now Kahaani 2 has to be a huge...big film. And I will make it only when we have a really good script in place. Till we are not sure and satisfied about the storyline we will not go ahead," Sujoy said.
After two back-to-back flops in Home Delivery and Aladin, Ghosh tested success with the Vidya Balan-starrer Kahaani last year. He made his Bollywood debut in 2003 through Jhankaar Beats.