Is kazuo ishiguro's mother tongue Japanese or English?

Kazuo ishiguro was born in Nagasaki, Japan on 1954. When I was over five years old, my family moved to England because of my father's job transfer. According to kazuo ishiguro, his family didn't intend to emigrate at first, but by chance, he stayed here for 29 years and he hasn't been back to Japan.

As a second-generation Japanese immigrant, kazuo ishiguro grew up entirely under English education, and English is almost his mother tongue. As he himself said, his Japanese is "poor" and he almost never writes in Japanese.