The task you seek, like many things, isn’t as daunting as it appears to be if you have some understanding of what it entails. There are a few things you need to be aware of.
First, the method of keeping vital records in Japan is different than what we’re familiar with here. The U.S.system of keeping vital records over time has become one of separate records for every event for that individual’s life, usually kept in the place the particular event happened. In Japan they use a family registry system, records are kept in what is called a koseki. Each family has its own koseki. The koseki might contain as little as one generation to several depending how long the family has lived in the same place.
Secondly, you’ll need someone who can read and write Japanese for you.
Thirdly, you’ll need to be able to provide family lineage proof, i.e. how are you related to the person you’re researching.
Lastly, you’ll need to locate an immigration record, or record that tells when immigrated and possibly household name. In your case, Ryuichi Yoshimura appears to have first immigrated to Hawaii in 10/15/1903 aboard the S.S. Ceoptic with his wife. The immigration record will be the key. Since Ryuichi’s port of entry was Honolulu, and now that I've provided you with this information, you’ll next want to contact the Consulate General of Japan in Honolulu, Attn: Genealogy, 1742 Nuuanu Ave. Honolulu, HI 96817(808) 543-3111. You may want to call them to have them send you an immigration record request form. It is on this form you’ll be required to submit lineage proof, (I’m uncertain if census records are acceptable or only if vital records are required, something you may want to ask when you call them). I side stepped this by having my Aunt, who’s a direct descendant submit the form. The Japanese record differs from the U.S. record in that it usually shows the name of the householder and his family members, their relationship, birthday, date of entry to Honolulu and a domicile address (at the time). It will be in Japanese. If you’re able to have it translated, (where I’m stuck at right now), the two critical bits of information you hope to learn is the householder name and domicile address. Once you know this, you’ll want to get back in touch with the Consulate General asking them to help you determine the local government office in Japan you’ll need to contact to obtained his koseki. (Since the war, Japan has re-districted several times, and most likely his village no longer exists).
After accomplishing all of this, you’ll want someone to write in Japanese, (best results I understand), a letter asking what fees and other requirements, i.e. records needed and if they have to be certified and in Japanese, (the Consulate maybe able to answer the latter), in requesting the koseki desired. I also understand it also helps if the payment is made in their currency. Again, another question you may want to address to the Consulate General.
Best of luck to you, Ryan.
P.S. I’d be interested in hearing if you’re successful and other procedures or things you may have learned along the way that I’m not aware of or simplifies the process.