Thermodynamic properties of 'ideal three-phase model' are studied using the result of Monte Carlo calculations of Hoover et al. and are compared with experiment. The ideal three-phase model is characterized by the pair potential φ(r) = C/rn - αγ3 exp (-γr), where n(>3), C(>0) and α(>0) are the parameters specifying the substance in the scheme of the model. The positive constant γ is the quantity to be made to tend to zero after taking the thermodynamic limit. It is found that the melting equations of solidified inert gases are well represented by putting n ≃15. Systematically different melting behaviours of alkali metals from solidified inert gases can be accounted for by assigning the former the smaller value of n such that 4 ≪ n ≪ 5. The implication of the difference of the value of n in the two families of substances as well as the limit of the applicability of this model is discussed.