To reduce the amount of dietary crude protein (CP) and nitrogen excretion, this study aimed to improve the feed for fattening Japanese Black steers. The dietary CP content was 12%-13% on a dry-matter basis in the control diet (CON, n = 4, initial body weight [BW] 304 kg) and 11%-12% in the moderate-protein diet with rumen-protected lysine and methionine during the early and middle fattening stages (LysMet, n = 4, initial BW 297 kg). BW, BW gain, intakes of dry matter and total digestible nutrients, and the gain-to-feed ratio were not affected by dietary CP content. CP intake was significantly lower in LysMet-fed steers than in CON-fed steers (p < 0.01); however, there were no differences in the intake of metabolizable lysine or methionine between groups. Total nitrogen excretion was lower in LysMet-fed steers than that in CON-fed steers at the early (p < 0.10), middle (p < 0.05), and late (p < 0.10) fattening stages. Carcass weight and traits were not affected by the dietary CP content. These results indicate that in Japanese Black steers, 11%-12% CP diets supplemented with rumen-protected lysine and methionine reduce nitrogen excretion without affecting productivity.