Sunil Giri attacked his family members, mostly women and children, at his home at Dihjadi village late last night when the other male members were away to attend a marriage, the police said.
Using an axe, he attacked and killed on the spot his 65-year-old mother, a seven-year-old niece and a five-year-old nephew.
His two other victims - a 30-year-old sister in-law and a 10-year-old niece succumbed to their injuries at Patna Medical College and Hospital (PMCH).
Senior police officials rushed to the spot to investigate and a manhunt has been launched to net Giri.