-- From the BBC World Service
The 72-year-old empress was suffering from intestinal bleeding, nose bleeds and mouth ulcers, an agency spokesman said.
Empress Michiko will take a short break from her duties to recuperate.
Her daughter-in-law, Princess Masako, has been suffering from a stress-linked illness for several years.
Masako, a former diplomat, has been seen in public only rarely since late 2003 because of what the palace calls an "adjustment disorder".
Her troubles have sparked debate about the lives of royal women within the confines of the very traditional and conservative Imperial Household Agency.
Empress Michiko fell ill with a cold on 19 February, the agency said, and had not felt well since then.
A spokesman for the agency outlined her symptoms at a news conference.
"Her doctors believe this is due to psychological stress," the spokesman said.
She will take a five-day break from her duties before receiving the Swedish king and queen. She will then spend more time recuperating outside Tokyo.
Empress Michiko, who is the wife of Emperor Akihito, was the first commoner to marry an heir to the throne.
But she has suffered from stress in the past and in the 1990s lost her voice for a period of several months, apparently after unflattering reports in tabloids.
Earlier this month, the Japanese government complained about a book by an Australian journalist about Princess Masako and her life as a royal, saying it insulted the royal family.