How to find my ancestor relative in Japan?

My grandmother was married to a japanese officer on the Japanese invasion in the Philippines during WW II. His name is Salustiano Nomata, but now they're both dead and no one from our elders knows my grandfather family. To where should I start if want to search my family root from japan?

Before you can even start, you need to get the name right. Salustiano is not a Japanese first name & the surname Nomata may not be correct. I suggest to obtain the name in kanji spelling first and find any other info. such as where he came from, i.e. the region, prefecture, city, town, village, etc., relatives, friends, etc. There are some outfits/detectives to search but I don't know how badly you want to search.

Your grandfather must have had his own military handbook (called gunjin-techo) which tells his rinte name, ranking, whcih troops (navy or army?) he belong to, plus maybe the birth place, etc. If you can find it in his articles left, you can easily find who he was.

His first name you gave is not his original. Last name Nomata can be either 野又、野俣 or 野亦. It reads No-mata. There are not so many like Suzuki or Honda, but a common name.

I think he was belonging to one of the troops who were acting in your country as of or around August 15, 1945. And it should not be difficult to find the list of such troops.