While I have no clue about the workings of the Japanese Immigration Department, I'd say that logically, your husband-to-be should apply for a spousal visa for you.
And I'm just guessing, but I'd think that you'd need to be officially married and have a nice set of pictures of the festivities and your married life to prove your marital relationship to the bureaucrats we all love, before you can even apply for a spousal visa.
Hope my two cent help.
Remixer