Abstract: The apparatus for filleting fish comprises guiding flaps arranged in succession and a system of knives. Said guiding flaps are adapted for feeding and orienting the fish which travels in the process of treatment. Said system of knives consists of circular knives and two filleting knives. Said circular knives adapted for cutting out a strip of flesh on the bottom of the fish belly are disposed behind the guiding flaps. Said two filleting knives are mounted one opposite the other. Disposed between the cutting edges of said filleting knives and the circular knives is a flattener. Said filleting knives have a ring form and are mounted on carrying rollers. Said cutting edges of the filleting knives are set in the direction facing the movement of the fish and coaxially with the side surfaces of said flattener. Said side surfaces of the flattener have a cylindrical form in the zone of said cutting edges.