Simply marrying a Japanese citizen does not automatically grant Japanese citizenship to a foreign national. Attaining Japanese citizenship must be done though the normal naturalization process. However, marrying a Japanese citizen does allow the foreign national to immediately obtain a spousal visa.
Can you get Japanese citizenship through marriage?
If you are a lawful permanent resident, your new spouse becomes a preference relative, in category F2A. That means your Japanese spouse can obtain permanent residence in the U.S. only after a visa number (space for another permanent resident) has become available.
What happens when you marry a foreign citizen?
An immigrant who marries a U.S. citizen must apply for a green card (U.S. permanent residence). This is a long process involving many forms and documents. After successfully obtaining a green card, the immigrant spouse can, after three years as a permanent resident, apply for U.S. citizenship.