An optimal planning method is proposed for the renewal planning problem of an energy supply system installed into building from economic viewpoint. In this method, the objective function to be minimized is the average annual total cost during the evaluation period of the objective system based on the annualized costs method, and the renewal year is considered as a decision variable together with the renewal system’s structure, equipment capacities, and system’s operational strategy. This problem is formulated as one of the mixed integer linear programming problem, and a numerical study is carried out for an office building with the total floor area of 15 000m2 by using the GAMS/CPLEX solver. Impact analysis of the carbon tax introduction is mainly studied on the renewal planning mentioned above, and the following results are obtained:

a) The optimal renewal strategy is not affected by the future carbon tax introduction at the standard rate such as 655 Japanese yen/t-CO2 argued presently in Japan.

b) By increasing the above tax rate more than 13 times higher, structure of the optimal renewal system changes suddenly, and equipment with high energy conversion efficiency becomes to be installed in spite of its expensive initial cost.