The interaction behavior between a monolithic low dielectric block with unidirectionally aligned through holes and an electromagnetic wave at a frequency range from 18 to 40 GHz has been studied. Hexagonally aligned through holes, whose diameters are 8.0, 9.0, and 10.0 mm, are introduced to a polymethylmethacrylate block. The electromagnetic wave reflection and transmission spectra perpendicular to the hole axis show a unique structure dependence, which is related to the diameter of the hole and its arrangement. A large decrease in the reflectance and transmittance appears in the spectra, which originates from the interference effect between the electromagnetic wave and material. It is concluded that the material has a potential for controlling the electromagnetic wave at a tailored target frequency and is expected to be usable as monolithic low dielectric electromagnetic wave absorbing material.