Reflecting optical element attachment mechanism and projection display device

An attachment mechanism (10) is characterized by comprising a mirror base (11) provided with a first adhesive portion (13c, 13d, 13e) and fixed to a device and a mirror holder provided with a second adhesive portion (14c, 14d, 14e) and holding a reflecting mirror (203a), and in that one of the first adhesive portion (13c, 13d, 13e) and the second adhesive portion (14c, 14d, 14e) is provided with a filling space that is capable of being filled with an adhesive while housing at least part of the other.