Abstract: A large-size spherical concave mirror, comprises a plurality of small-size concave mirrors joined together to constitute the large-size spherical concave mirror. Each of the small-size concave mirrors includes a concave mirror surface and at least three joining surfaces extending from the concave mirror surface, the small-size concave mirrors adjacent to each other being joined together through the joining surfaces, each of the joining surfaces and a spherical center of the large-size spherical concave mirror being disposed on a respective plane so that the joining surfaces do not face to the spherical center of the large-size spherical concave mirror.