The boy was playing under the bed in the house of Israil Khan at Nowpara Bandharpur village under Bagnan police station area when a bomb kept under it exploded, police said.
Local people admitted the boy to the Bagnan hospital in a critical condition as he bore multiple splinter injuries all over his body, they said.
A police contingent went to the area for an investigation, but found the room where the blast took place locked.
Police did not find any resident of the house around.
It also found bomb-making materials including strings to tie up a hand bomb strewn in the room.