A zoo employee in Nasu, Tochigi Prefecture, was listed in serious condition Monday after a lion bit him in the head and chest, police said.
The 14-year-old lioness attacked Yasuki Uchiyama, 21, at around 11:15 a.m. while he was cleaning a fenced-off area for lions at the Nasu Safari Park, they said.
Uchiyama was immediately taken to a nearby hospital. The chest wound is classified as serious, according to the doctors.
The zoo was open when the accident took place.
It was the second such accident at the zoo involving lions. In November 1997, a female employee and an intern at the zoo were mauled by a different lioness during a feeding session and suffered serious injuries.
After the 1997 accident, local authorities sent investigation papers on the operators of the zoo to prosecutors on suspicion that they violated labor safety regulations.
The zoo said it has since established detailed manuals for workers assigned to feed carnivores.
The zoo was opened in 1978, and currently about 700 animals of some 70 species roam freely within fenced areas, where visitors see the animals from inside vehicles.
Uchiyama joined the zoo in 1998 and was assigned to care for lions.
In Monday's accident, Uchiyama was working with two female employees cleaning up a passageway near some animal cages when the lioness suddenly attacked him. The two other workers were not injured.
Lions at the zoo are kept in several metal cages. The cage for the lioness in question was not locked at the time of the accident.
Police plan to question zoo operators to find out why the cage was left unlocked.