In this study, a method to measure the reflectance of a radio wave absorber, one of the performance evaluation items of an anechoic chamber, was studied. If the radio wave absorber is attached to the wall of the shield room, it can absorb electromagnetic waves traveling toward the wall and reduce reflection, resulting in a radio wave environment similar to free space inside the shield room. Radio waves that are not absorbed by the installed radio wave absorber become reflected waves, affecting the test results performed inside the room. In this study, a more efficient measurement method is proposed by comparing the reflectance measurement method using the frequency domain and the reflectance method using the time domain.