Marriage certificate for Japanese female and British man

How do I trace and get a copy of a marriage certificate between a Japanese female national and a British man?

The marriage took place sometime between 1975 and 1980. I have the full name of the British man but only the given name of the Japanese lady. Thank you for any help in finding this.

Where did they get married?

Which country was the marriage?

Assuming (from the question) that the marriage took place in Japan (Japan law-based marriage), you would need to know which municipality (city, prefecture) the marriage took place, and the full name of the Japanese national, preferably also with a more specific timeframe. Also note that you must be a relative or family member to be able to be able to obtain such a document/access family registry ("koseki"). Normally it takes the Japanese spouse to do it - it will be the Japanese national's "koseki," with her marriage to the non-Japanese spouse noted in her "koseki," I believe.