Meikyuu Kingdom: Damu Tokushu Butai SAS no Ossan no Isekai Dungeon Survival Manual!

Expert soldier Taiga has just retired from the military after twenty years of service. Listless, he decides a visit to his homeland of Japan will help him figure out what to do next. But he never arrives. A strange earthquake in the airport seems to spell his demise, but instead it transports him to Million Dungeon, a labyrinthine world of caverns, kingdoms, magic...and monsters. The key to survival is apparently creating his own nation, but even with all of Taiga’s experience, the incredible dangers may prove too much...

(Source: Yen Press)