About 5 minutes walk from Oura Tenshudo-shita Station Nagasaki Electric Railway.
The official name is "Church of the 26 Japanese Martyrs". Completed in 1864. It is the oldest church in Japan, and has become a National Treasure. Is very close to the entrance of Glover Garden.
These buildings are included on "Churches and Christian Sites in Nagasaki" where is the Tentative list in UNESCO World Heritage Site.
This building is situated right next to the Oura Catholic Church. Was built in 1875.
It is now open as Christian reference room.
Google Map Oura Catholic Church
Japanese site is here.