Fourteen other policemen were injured, five of them seriously.
The blast occurred when a police patrol tried to open a box which they had been told contained documents about Maoist activities, in the state's remote Chatra district, 200km northwest of capital city Ranchi.
"Police had gone there after a tip-off. They found this box which was actually a bomb. It now looks like a trap," police superintendent Sashinath Jha told Reuters.
There were no immediate claims of responsibility for the attack.
Maoist rebels, who claim to be inspired by Chinese revolutionary Mao Zedong, have been fighting for land and jobs for agricultural workers and the poor.
The rebels, active in five southern and eastern Indian states, often target police, accusing them of colluding with rich landlords.
In August, India's Home Ministry said there were about 9300 armed Maoist rebels in the country. Officials say rebels have joined hands with Maoists fighting to overthrow the monarchy in neighbouring Nepal.