To reliably adhere a photocurable resin and a dentin by solving a failure and the shortage of mechanical strength caused by polymerized contraction and irradiating the photocurable resin packed in a root canal with light from inside so as to polymerize-cure the photocurable resin.

The apparatus comprises: a metallic tube 1 provided with a plurality of holes 2 at its side wall; an optical fiber 3; and a light source. The distal end site of the optical fiber 3 is inserted into the hollow part of the tube 1, and the proximal end of the optical fiber 3 is connected optically with the light source. Furthermore, the distal end site of the optical fiber 3 is constituted so as to leak light from an outer surface. The resin 7 is cured by bringing the outer surface of the tube 1 into contact with the resin packed in the root canal and irradiating the resin 7 with light from the holes 2.