In this research, an environmental assessment index considering the wellness of workers is proposed by comparing the evaluation items of the WELL Building Standard and Comprehensive Assessment System for Built Environment Efficiency (CASBEE)-the Japanese building environmental assessment index-in order to investigate and improve the health performance of Japanese offices. Based on the results of the comparison, “indoor environment,” “functionality,” “nourishment,” and “fitness” were selected as the main evaluation items of the proposed index. Furthermore, to set the weighting of each evaluation item, a questionnaire on health awareness was conducted among office workers. Four office buildings were investigated using the proposed index, WELL Building Standard, and CASBEE. A comparison of the evaluation results for the proposed index and CASBEE indicates the possibility of evaluating healthy performance and sustainability through a combined use of the two indexes.