The pontiff called "indescribable" horror suffered by the victims of the atomic bombing in August 1945.
"This place makes us deeply aware of the pain and horror that we humans are capable of causing to each other," said the Pope.
He also emphasized that possession of nuclear weapons and other weapons of mass destruction is not an appropriate response to the search for peace.
"The exchange took place between Russia and Ukraine last week. I'm glad for the released people who could hug their relatives again and continue praising for ending the conflict and lasting peace," the Pope said.