The degree of conversion (DC) of dual-cured composite luting materials for composite inlay restoration was determined by Fourier transform infrared spectroscopy. Dual-cured composite cements showed a DC of 59.3-75.0% after self-cure and 66.6-81.4% after photo-cure. DC after photo-cure decreased with the increase in thickness of a restoring composite inlay between the composite cement and a light tip. A lower DC after photo-cure did not satisfactorily increase following subsequent self-cure. Sufficient light should be provided to the composite cement in photo-cure luting of composite inlays to achieve a high and uniform DC, as well as in filling restoration using photo-cured composites. Reirradiation, in which the tip of a light unit was brought close to the portion of the cement with a low DC, was very effective in increasing DC and making it more uniform.