A guiding principle for developing practical thermoelectric materials was constructed on the basis of simulations of thermoelectric properties using linear response theory. Al-Mn-Si C54-phase, Al-Mn-Si C40-phase and higher manganese silicide (HMS), all of which consist solely of cheap, environmentally friendly elements, were selected using the guiding principle. The validity of the strategy to develop practical thermoelectric material was clearly proved by our newly prepared HMS possessing a dimensionless figure of merit exceeding unity.