A method produces a cycloalkanone oxime by photonitrosation using light emitting diodes as the light source for the next generation that replaces an electric discharge lamp with, for example, mercury or sodium sealed therein. The method produces the cycloalkanone oxime by a photochemical reaction of a cycloalkane with a photo nitrosating agent in a liquid by light irradiation. The method uses a light source satisfying conditions that, in an emission energy distribution with respect to wavelength of the light source, a wavelength at which emission energy has a maximum value is in a range of 550 nm to 700 nm and a wavelength range outputting energy of or over 5% strength of the peak strength is equal to or less than 150 nm. An irradiation distance in the liquid is equal to or greater than 200 mm, and concentration of the photo nitrosating agent in the liquid is 0.1 mol % to 0.5 mol %.