A Japanese nurse admitted to killing at least 20 elderly patients and said she planned their deaths to occur when she was off-shift — because she didn’t want to be bothered explaining their demise to relatives during working hours.

Ayumi Kuboki allegedly poisoned the patients by lacing their intravenous drips with toxic antiseptic chemicals at specific times, Japanese newspaper the Asahi Shimbun reported Tuesday.

The 31-year-old, who worked at Oguchi Hospital in Yokohama, was busted Saturday on suspicion of murdering 88-year-old Sozo Nishikawa in September 2016.

She’s also being investigated for the death of Nobuo Yamaki, 88, two days later. Yamaki shared a room with Nishikawa on the fourth floor in the Kanagawa ward of the hospital.

While in custody, Kuboki admitted to police that she killed the two men — and “about 20 other patients” — over the course of two months in 2016 so she wouldn’t have to explain the circumstances of their deaths to their families.

She claimed she only targeted seriously ill patients who she believed were going to die on her shift, but sources said some of the victims were not in serious condition when they died.

“It would be troublesome if that responsibility fell on me,” Kuboki said.

Police believe the cold-hearted caregiver put cleaning product containing benzalkonium chloride into patients’ IVs — which left Yamaki and Nishikawa dead within hours.

The chemical, an antibacterial that’s readily available within hospitals, was found in Yamaki’s body and his IV bag.

Police ruled that his death, along with Nishikawa’s, were due to poisoning.

In his case, Kuboki allegedly sneaked into his room between 3 p.m. and 4:55 p.m. Sept. 18, 2016, ahead of her 5 p.m. night shift.

Nishikawa’s health declined shortly after and he died at 7 p.m. A nurse from the day shift broke the sad news to his family.

Police found the same solution in the bodies of two other senior patients, an 89-year-old man and 78-year-old woman, who died around the same time as the two men.

Staffers told Asahi Shimbun there were no red flags.

“She was the kind of person who was hard to figure out what she was really thinking, but she was considered competent,” a worker said.

But between April and August 2016, the fourth floor was plagued by a series of unsettling problems — including ripped nurses’ uniforms and drinks that were found spiked with unknown substances.

Police began investigating following Nishikawa and Yamaki’s deaths and found 10 unused antiseptic-laced IV bags with tiny holes in the rubber seal — indicating they were punctured with a syringe.

When they examined all of the nurses’ uniforms, they found the chemicals around a pocket on Kuboki’s uniform.

Police questioned Kuboki in late June after she quit the hospital. She admitted to killing her patients on the second day of questioning.

Kuboki earned her nursing license in 2008 and worked at another hospital before coming to Oguchi in May 2015.

In December, she eerily told Asahi Shimbun: “I have worked hard to help patients die in peace, so I was shocked by the incidents.”