Recently proposed two methods such as Quantum Monte Carlo Diagonalization method in nuclear structure physics and path-integral renormalization group method in condensed matter physics have a similarity as quantum many-body approach, especially in description beyond mean-field approximation. As these two methods have been developed in different fields, there are significant differences as quantum-number projection technique in former method and extrapolation technique in latter method. As these techniques are independent of details of considered system, these can be applied to other fields. In this paper, we discuss how to apply such extrapolation technique to shell model calculations and discuss how to apply quantum-number projection to path-integral renormalization group method.