The preliminary investigation suggested that the bomb was placed by unidentified criminals inside a motorbike near the main gate of the court in Sasaram — the headquarters of Rohtas district, a police official said.
The injured have been admitted to a hospital for treatment, police said.
The security in courts across the state has been beefed up following the blast.
Police have lodged a case in this connection and begun an investigation into it.
In March this year, a bomb exploded in Sasaram court premises.