Following an introduction by Vice President Mike Pence, Hagerty took the oath of office, with his left hand on a Bible held by his wife, Chrissy.

"I look forward to the challenge ahead of us," Hagerty, 57, said before an audience. "We're going to make a wonderful contribution to what is already a wonderful relationship."

Noting that now is "a momentous time" for the bilateral alliance, Pence said, "The United States and Japan stand together resolutely to confront whatever threats are posed to us, especially those posed by the regime in North Korea."

"You will help make the extraordinary friendship and alliance between the United States and Japan even stronger," he told Hagerty.

The swearing-in ceremony was joined by Hagerty's family members, friends and other guests, including Bob Corker (R-Tennessee), chairman of the Senate Foreign Relations Committee, and Katsuyuki Kawai, special adviser to Japanese Prime Minister Shinzo Abe.

A native of Tennessee, Hagerty spent about three years in Japan working for the Boston Consulting Group, before he launched investment firm Hagerty Peterson and Co. in the mid-1990s.

Hagerty is expected to arrive in Japan to assume the post as early as mid-August, according to informed sources. (Jiji Press)