A compound that exhibits excellent color tone stability and physical properties as well as excellent photopolymerization activity over a wide region from near-ultraviolet to visible region, permitting relaxed operation under ambient light, so that wide application can be found in the dental field and photopolymerization industry; and a relevant photopolymerization initiator and hardenable composition. In particular, there is provided a novel camphorquinone derivative having an acylphosphine oxide group [—(C═O)—(P═O)