彼はその分野の第一人者として知られている。
Kare wa sono bun'ya no daiichininsha toshite shirarete iru.
He is known as the leading person/authority in that field.
第一人者の講演を聞く機会を得た。
Daiichininsha no kouen o kiku kikai o eta.
I had the opportunity to hear a lecture by the leading authority.