Digital Terrestrial Television Broadcasting (DTTB) system adopted in Japan, uses Orthogonal Frequency Division Multiplexing (OFDM) modulation, and Spectrum Mask for OFDM modulation waves is defined. This Spectrum Mask is defined to coexist with adjacent analog television signal and adjacent digital television signal. When applied to the transmission network for relay system, the spectrum mask must be more stringent in performance depending on the required transmission characteristics. The effect exerted by the spectrum mask on the transmission characteristics of the adjacent channel is represented by the equivalent C/N. The relationship between the required transmission characteristics and the spectrum mask is studied, and findings obtained are described below.