We recently developed and characterized an absorber for millimeter wavelengths. To absorb a millimeter wave efficiently, we had to develop a low reflection and high absorption material. To meet these requirements, we added polystyrene beads in the epoxy for multiscattering in the absorber. The typical diameter of polystyrene beads corresponded to the scale of Mie scattering for photon multiscattering in the absorber. The absorber consists of epoxy, carbon black, and expanded polystyrene beads. The typical size of the expanded polystyrene beads is consistent with the peak of a cross-section of Mie scattering to increase the mean free path in the absorber. By applying this effect, we successfully improved the absorber’s performance. In this paper, we measured the optical property of epoxy to calculate the Mie scattering effect. Based on the calculation results, we developed eight types of samples by changing the ratio in the absorber material. To compare the eight samples, we characterized the reflectance and transmittance of the absorber in a millimeter wavelength. The measured reflectance and transmittance of a 2 mm thick sample with optimized parameters are, respectively, less than 20% and 10%. We also measured the transmittance in a submillimeter wavelength. The measured transmittance is less than 1%. The shape of absorber can be modified for any shape, such as chip and pyramidal shapes. This absorber can be used to mitigate the stray light of a millimeter wave telescope with any shapes.