The invention provides a process for efficient synthesis of ammonia. The process includes a water-splitting step in which water is decomposed to obtain a first source gas comprising hydrogen and oxygen, a first oxygen removal step in which the oxygen contained in the first source gas is at least partially separated and removed by an oxygen separating membrane or hydrogen separating membrane to obtain a second source gas having a lower oxygen concentration than the first source gas, a second oxygen removal step in which the oxygen in the second source gas is reacted with hydrogen to produce water for removal, or is adsorbed onto an adsorption medium for removal, to obtain a third source gas having a lower oxygen concentration than the second source gas, and an ammonia synthesis step in which the hydrogen in the third source gas is reacted with nitrogen to synthesize ammonia.