Acting Chairman of the Bihar Legislative Council Awadhesh Narayan Singh tested positive for COVID-19 on Saturday.
His wife, two sons and daughter-in-law have also tested positive for the virus, said Singh's personal assistant Rahul Kumar who was also found to be infected.
Awadhesh Narayan Singh was admitted to the isolation ward of AIIMS-Patna for treatment after his test report came in, the hospital's nodal officer on COVID-19 Sanjeev Kumar said.
His wife Meena Singh, elder son Anand Raman and personal assistant Kumar are also being admitted to AIIMS, while younger son Nishant Ranjan and daughter-in-law Khushbu Singh will be in home isolation, family sources said.
"The test results of the six persons, including myself, came today and the reports of other family members are likely to come by late evening," Rahul Kumar said over the phone.
Awadhesh Narayan Singh and all his family members had fever, following which samples were collected on Friday by a team of the Indira Gandhi Institute of Medical Sciences.
He administered the oath of office to nine newly-elected members of the Legislative Council on July 1.
Chief Minister Nitish Kumar, Deputy Chief Minister Sushil Kumar Modi and a host of other political leaders and officials were present at the oath-taking ceremony.
The chief minister has also sent his swab sample for COVID-19 test after Awadhesh Narayan Singh was found to be infected with the virus.
He is the fifth legislator in the state to have tested positive for COVID-19.
Minister Vinod Kumar Singh and his wife have tested positive for the pathogen, besides BJP MLA Jibesh Kumar Mishra, Congress MLA Anand Shankar Singh and RJD MLA Shahnawaz Alam.
Veteran RJD leader Raghuvansh Prasad Singh, a former Union minister, has been discharged from AIIMS after recovering from the disease.