The Basilica of the Twenty-Six Holy Martyrs of Japan also Ōura Church is a Roman Catholic minor basilica and Co-cathedral in Nagasaki, Japan, built soon after the end of the Japanese government's Seclusion Policy in 1853. It is also known as the Church of the 26 Japanese Martyrs. It was for many years the only Western-style building declared a national treasure, and is said to be the oldest church in Japan.