The attack happened in the early hours of Tuesday. Another 45 people were reportedly injured, 18 seriously.
Local police have arrested a male suspect after he turned himself in.
The man is said to be aged 26, and is a former employee at the the Tsukui Yamayuri Garden facility, 40km (25 miles) south-west of Tokyo.
He reportedly went into a police station and said “I did it”. No motive has been established yet.
Details are still emerging about the incident, but the death toll makes it the worst mass stabbing in Japan in recent years.
In 2008, a man drove a truck into a packed shopping district at Akihabara in Tokyo, before climbing out and randomly stabbing people. Seven people died.
The attack occurred on the same date that a man with a history of mental illness stabbed eight children to death at an Osaka primary school in 2001.