In the ammonia synthesis method, an anode and a cathode are arranged in an electrolyte phase at a predetermined interval; water (H2O) is supplied to an anode zone and light is radiated so that water is decomposed by a photoabsorption reaction to generate protons (H+), electrons (e−), and an oxygen gas (O2); a nitrogen gas (N2 ) is supplied to a cathode zone, and the electrons (e−) generated in the anode zone are allowed to transfer to the cathode zone through a lead, thereby generating N3− in the cathode zone; and ammonia (NH3 ) is synthesized through the reaction between the protons (H+) that have moved toward the cathode zone from the anode zone in the electrolyte phase and N3−.