An evaluation method of platinum resistance thermometers (PRTs) used in the stationary air was considered. A comparison measurement apparatus to evaluate PRTs was developed by using a commercially available temperature bath and the performance of the apparatus was evaluated. The apparatus has high temperature stability and good temperature distribution although the thermal medium is the air. The uncertainty of the apparatus is less than 1mK. A PRT was evaluated for a trial with this apparatus at 23°C. First, a PRT that was set in the water was measured with a Standard Platinum Resistance Thermometer (SPRT) in the temperature bath. Next, the identical PRT was set in the air chamber of the developed apparatus and measured with the SPRT. And, each result was compared. As a result, it was possible to distinguish the difference of behavior of the PRT at different environment by a proposed method in this study. And, it was confirmed that the cause of different behavior was the self-heating of the PRT.