Japan's ageing Emperor Akihito was admitted to hospital on Saturday for a heart examination after showing symptoms of a restricted blood flow to his heart amid increasing worries about his health.

The 78-year-old monarch was scheduled to undergo an angiogram of his coronary arteries and stay overnight at the University of Tokyo Hospital, the Imperial Household Agency said.

He was accompanied by Empress Michiko to the hospital Saturday morning.

The test was arranged after an electrocardiogram showed restricted blood flow to his heart, the agency said.

He also had an angiogram last year at the same hospital, which found his arteries were narrowing and he was put on medication.

In November, the emperor was hospitalised for 19 days for mild pneumonia.

Akihito, who ascended the throne in 1989 following the death of his father Hirohito, underwent surgery for prostate cancer in 2003.

The emperor's second son Prince Akishino in November suggested there should be a discussion about setting a retirement age for the titular head of state.

Despite being stripped of much of its mystique and its quasi-divine status in the aftermath of World War II, the Japanese throne is held in deep respect by much of the public.