Wedding preparations are underway for Lower House member Shinjiro Koizumi, 38, a prominent politician who is widely seen as a potential future prime minister, and TV personality Christel Takigawa, 41.
The pair made the surprise announcement at the Prime Minister's Office on Wednesday. Their relationship was until that moment a closely guarded secret.
Separately, Koizumi wrote on his blog that Takigawa is pregnant and due to give birth early next year.