A nanocrystal production method includes a light irradiation step of applying light to a surface of a metal material immersed in water to form nanocrystals on the surface. In this nanocrystal production method, the metal material contains iron, the nanocrystal contains at least one of iron oxide and iron hydroxide, and in the spectrum of the light, a wavelength at which the intensity is maximum is not less than 360 nm and less than 620 nm.