To improve the cryocooler efficiency, the regenerator performance should be well understand. As one of the evaluation methods to know the regenerator performance, in general, the regenerator efficiency which is defined as the ratio of actual heat transfer rate to the maximum possible heat transfer rate has been used. In the case of the regenerator for the cryocooler, however, the definition of the “maximum possible heat transfer rate” does not clear because of the significant effect of the working gas within the void volume in the regenerator. In this study, the basic equations of the regenerator having the void volume has been introduced. These equations are applicable for the boundary condition of the time dependent mass flow rate and pressure. A set of the numerical solutions of these equations for the ideal gas indicates that the mass flow at the low temperature side of the regenerator is always greater than that of the high temperature side, which leads the uncertainty of the maximum possible heat transfer rate. An alternative expression of the regenerator efficiency has been proposed.