[en] Double beta decay of nuclei is an interesting problem which concerns with particle physics and nuclear physics. For example, the neutrinoless double beta decay (0νβ) provides a good test of the finite mass of the neutrino and the existence of the right-handed weak charge current. In most of the analysis of the experimental results, the conclusions derived strongly depend on the calculated nuclear matrix elements for the double beta process. Therefore, precise calculations of the matrix elements are highly desirable both for the 0νβ and 2νβ process. In this paper, the authors will point out some problems in standard methods of evaluating the nuclear matrix elements of the 2νβ decay and propose a new practical method. The method is applied to the ⁴⁸Ca to ⁴⁸Ti 2ν double beta decay