Fire fighters are working under severe conditions. To secure the work safety of fire fighters, objective indices of fatigue are proposed based on frequency analysis of cardiopulmonary data under heavy duty. We especially focused on the shape of the power spectral density with respect to the frequency and fluctuation index in addition to the self-judgment of each personnel and experience in the field commander. These indices include more information than heart beat rate and give better understanding of the fire fighters’ cardiopulmonary condition and have a possibility to become good tool to prevent them falling into the severe acute fatigue caused by fire-fighting operation.