Emperor Emeritus Akihito, 88, has been diagnosed as suffering from heart failure, but his condition has improved after receiving treatment, the Imperial Household Agency said Tuesday.
The father of Emperor Naruhito received the diagnosis due to a faulty heart valve after undergoing checks at the University of Tokyo Hospital on Sunday, the agency said.
Heart failure is a condition in which the heart muscle does not pump blood as well as it should and can be treated. It differs from a heart attack, where blood flow to the heart is suddenly blocked.
The former emperor has been undergoing treatment with medication and through restricting activity and fluid intake since the end of June, the agency said.
"Currently he is living life as usual," an agency official added.
He stepped down from the Chrysanthemum Throne on April 30, 2019, becoming the first Japanese monarch to abdicate in around 200 years. His eldest son, Emperor Naruhito, succeeded to the throne the following day.
The former emperor had indicated his wish to relinquish the throne due to concerns over his advanced age and declining health, and it was granted through a one-off piece of legislation.