The bomb kept on the bike of Sachin Kumar, parked outside the court premises, exploded after he started the engine, Assistant Superintendent of Police Sushant Saroj said.
Kumar is suspected to have accidentally pressed the detonator that he was carrying along with the explosive, he said, adding the detonator had been recovered from the spot.
The seriously injured man was taken to the Sadar hospital where he died during treatment, the ASP said. Two other persons, Shivratan Bind and Shri Paswan, suffered splinter injuries, Saroj said. Senior police officials have gone to the spot.