The health ministry is calling for vigilance at hospitals across the country after the first case of HIV type-2 infection has been confirmed involving a Japanese person.
The Health, Labor and Welfare Ministry received a report last month from a hospital that a man had tested positive for HIV-2 after receiving a blood transfusion in a West African country.
The HIV-2 strain differs from the more prevalent HIV-1 virus.
The man underwent HIV tests after being hospitalized for asthma. He was released from the hospital after receiving treatment, officials said.
The ministry believes the man was infected with HIV-2 because of its prevalence in West Africa.
Three previous cases of HIV-2 infection have been confirmed in Japan, but they involved male foreign nationals.
Hospitals and health centers in Japan typically conduct HIV tests in two phases. The preliminary test screens for all types of HIV infection and the second confirms the infection and determines the strain.
A ministry official said both tests are usually performed in HIV screening but called for medical facilities to be on the lookout for HIV-2 infections, even though they are rare in Japan.
The ministry has sent letters to prefectural governments, asking them to alert local hospitals and health centers about the possible spread of HIV-2 in Japan.
HIV-2 has a different genetic makeup from HIV-1 and is believed to be less infectious.