According to the present invention there is provided an ammonia synthesis method using solar thermal energy, whereby it is possible to minimize the load of collecting solar thermal energy, and especially high-temperature solar thermal energy. The method of the present invention for synthesis of ammonia using solar thermal energy includes the following steps (a) to (c): (a) conducting ammonia synthesis reaction in which nitrogen and hydrogen are reacted to synthesize ammonia, (b) heating a heating medium by solar thermal energy and the reaction heat energy of the ammonia synthesis reaction, and (c) conducting at least part of the water splitting reaction in which water is split into hydrogen and oxygen, using the thermal energy of the heated heating medium, to obtain the hydrogen.