Multi-functionalty of agricultural water and irrigation and drainage systems of Korea was identified by AHP method with surveying expert opinions. Major function identified were ‘agricultural water supply', ‘multipurpose water supply', ‘environmental conservation', ‘fostering social culture' and, ‘disaster prevention and energy'. The function as agricultural water supply showed significantly higher weight value than the others. Rational share of operating and management cost for irrigation and drainage facilities is current issue in Korea. Relative benefit of each function of irrigation and drainage system to the beneficiaries was calculated and summed up. Based on this calculation, the rational ratio of cost sharing for management of the system by beneficiaries is proposed as follows(100% in total); people(nation) 40, region(municipalities) 25, local(village) residents 15, farmers 10, and other users 10%.