Hayato Imai, 23, was arrested yesterday on suspicion of killing Tamio Ushizawa at the nursing house in Kawasaki southwest of Tokyo in November 2014, according to a spokesman for the Kanagawa Prefectural Police Department.
Police suspect the victim died of ruptured organs after being hurled by Imai from the fourth-floor balcony, the spokesman said, adding that further details including his motive were not available.
The Asahi Shimbun daily and other local media reported that the suspect also admitted killing two more residents in their 80s and 90s by throwing them from balconies in 2014.
Imai was fired last year after being arrested on suspicion of stealing cash and jewelry from the room of a female resident at the facility.
The elderly are traditionally respected in Japan and there is even a national holiday reserved for honouring senior citizens.
Japan's population is rapidly ageing, with a quarter of the population currently aged 65 or over. That proportion is expected to grow to around 40 per cent over the next four decades.