Carbon-13 nuclear magnetic resonance spectra of thirty-two kinds of 5, 5-disubstituted barbituric acid derivatives were measured. Two conclusions were obtained from analyses of these spectra. (1) The additive law is applicable to chemical shifts of sp³-hybridized carbons. (2) Hammett's constant σ in Hansch analysis can be replaced by a quantity A expressed by the value related to chemical shifts of the 5-position carbon.