An ammonia synthesis apparatus includes: a first gas channel; a second gas channel disposed outside the first gas channel; a third gas channel disposed outside the second gas channel; an air supply unit that supplies air to the second or third gas channel; a water supply unit that supplies water to the first gas channel; and a heat supply unit that supplies heat to the first gas channel. A metal or a metal oxide that reduces water to produce hydrogen is placed in the first gas channel. An ammonia synthesis catalyst is placed in the second gas channel located downstream of the downstream end portion of the first gas channel. The second and third gas channels are at least partially partitioned by an oxygen permeation membrane, or a nitrogen permeation membrane, so that oxygen is supplied to the third gas channel, and nitrogen is supplied to the second gas channel.