It said her cancer had not spread, but she will have checkups to determine if further treatment is needed.
Michiko's husband, Akihito, abdicated as emperor on April 30 and their son, Naruhito, became emperor the following day.
Michiko is the first commoner to become empress in modern Japanese history.
Akihito and Michiko have been popular with the Japanese public for breaking with traditions and bringing many changes to the monarchy. They chose to raise their three children themselves, spoke more often to the public, and made amends for war victims in and outside Japan.
The emperor and empress have no formal political power and have only symbolic status.
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