Shigeaki Mori, 79, told reporters that Obama's speech last month in Hiroshima convinced him that it wasn't an empty promise.
"I thought he really meant it," Mori said, recalling his emotional encounter with Obama. "I thought he really cares about world peace and strives to achieve it in his lifetime, and was not just saying it to be nice."
He said he listened carefully to Obama's speech in Hiroshima on May 27, and to his earlier speeches including one in The Hague which helped him win the Nobel Peace Prize, and thought Obama is a man of principle and is serious about achieving a world without nuclear weapons.
Obama's speech generally won praise in Japan, but a survivors' group criticised him for not acknowledging any direct US responsibility.
Mori, an amateur historian, has devoted more than half of his life to piecing together the fates of a dozen US airmen who were held as prisoners of war in Hiroshima and were killed by the atomic bomb.
The US atomic bombings of Hiroshima and Nagasaki in the final days of World War II devastated both cities and killed more than 200,000 people.