Upper house lawmakers voted 94-2 to uphold Boonsong Kowawisarat's constitutional right to immunity during the parliamentary session.
"After the session ends on November 28 police can take him for questioning," Senator Somchai Sawaengkarn told AFP. "A senator should do his job during a parliamentary session."
Boonsong has said the death was an accident caused by ill health following a stroke. He said the gun went off while he was trying to remove a jammed bullet, fatally wounding Chanakarn Detkard, 46, who was also his secretary.
Police said Boonsong, who is the representative for northern Mae Hong Son province, is likely to be charged with causing death by negligence.
If convicted, he faces a maximum punishment of 10 years' imprisonment and a 20,000 baht ($670) fine.
Under the constitution, a lawmaker cannot be arrested, detained or summoned as a suspect in a criminal inquiry during a parliamentary session without permission from the house.