The semiempirical method was suggested for construction of a simple equation of state for liquid and gas. With this purpose. some new conditions and restrictions, which should be satisfied by the "right" equation of state. were formulated. In particular, the preferable structure of the equation of state was determined. The equations for description of thermodynamic properties or real gases and liquid phase were derived with involvement of additional restrictions. The second virial coefficient calculated on the basis of the modified step potential of intermolecular interactions was used as the temperature functions in the derived equations, and the density functions were determined from analysis of experimental data. A new equation of state for liquid and gas with six individual adjustable parameters was suggested. This new equation describes the thermodynamic properties of substances much better than the known Benedict-Webb-Rubin equation of state. Some caloric properties of substances were calculated.