Included are an ammonia synthesis column that synthesizes ammonia from a raw material gas, a discharge line that discharges a synthetic gas, a water-cooled cooler that cools the synthetic gas with a coolant, disposed in the discharge line, an ammonia separator into which a synthetic gas after cooling is introduced and which separates the ammonia gas and a liquid ammonia from each other, a raw material return line that returns a raw material gas containing the separated ammonia gas to the ammonia synthesis column side as a return raw material gas, and a compressor that compresses the return raw material gas, disposed in the raw material return line. An ammonia concentration in the return raw material gas is 5 mol % or more, and an ammonia synthesis catalyst that synthesizes the ammonia gas in the ammonia synthesis column is a ruthenium catalyst.