noun
Japan hand; expert on Japan
Refers to a non-Japanese person who is very knowledgeable about Japan, its culture, language, and customs. Often used with a nuance of deep familiarity gained through long experience.
彼は長年日本に住んでいる日本通だ。
He is a Japan hand who has lived in Japan for many years.
あの外交官は日本通として知られている。
That diplomat is known as an expert on Japan.
Compound of 日本 (Japan) and 通 (expert, connoisseur). The suffix 通 is commonly used to indicate a person well-versed in a particular field or place.