To improve characteristics of an already-existing radio wave darkroom at a low cost and with reliability without changing its construction.

A floor 1 of a radio wave darkroom comprises a metal plate 10. Its ceiling and side walls 3 are comprised of metal plates 11 as reflector, ferrite arranged inside of the reflectors and radio wave absorbers 13 arranged inside of the ferrite 12. In this case, the radio wave absorbers 13 are arranged on the side walls 3 which are parallel or approximately parallel to the measuring axis of the radio wave. Conductive material 20 is placed between adjacent radio wave absorbers 13. The site attenuation characteristics can be improved by the placement of the conductive material.