Many laboratory- and pilot-scale studies on anaerobic co-digestion have been conducted in Republic of Korea; however, studies on full-scale demonstration facilities are lacking. This study aimed to present a successful case of a large-scale anaerobic co-digestion facility in Republic of Korea for biogas generation from four organic wastes (pig manure, food waste, excretion, and thickened sewage sludge) using a horizontal anaerobic digester. A preliminary biochemical methane potential test was performed for the individual and mixed organic waste to design a treatment facility for 320 m3/day of organic waste generated in Seosan City. Subsequently, a horizontal anaerobic digester with a 35 day-retention time (based on 320 m3/day input) was constructed. Each organic waste was placed in an anaerobic reactor after pretreatment. The input was gradually increased after the first seeding, and the operation continued for 158 days. Total and volatile solids made up 4.1% and 3.3%, respectively. Throughout the operating period, the digester temperature was maintained at 35−40 °C for mesophilic digestion, and the pH was maintained at 7−8. The average organic matter removal efficiency (volatile solids basis) was 64% and the methane gas production rates were 0.35, 0.6, 0.26, 0.28, and 0.39 Nm3CH4/kg vs. for pig manure, food waste, excretion, thickened sewage sludge, and mixed waste, respectively, resulting in an average methane content of the biogas 68.8%.