The present invention provides a method for generating four-wave mixing to obtain idler light with high efficiency, in which the range of lengths of an optical fiber is appropriately set, and probe light and pumping light, having different frequencies, are launched into the optical fiber. When the nonlinear coefficient of the optical fiber, the loss per unit distance, and the wavelength and intensity of the probe light and pumping light are set to certain values, the idler light conversion efficiency at the output end of the optical fiber is a periodic function of an optical fiber having a maximal value and a minimal value. The maximum length of the optical fiber to be used to obtain four-wave mixing is set to be equal to or less than the length Lmax (Lmax=Lm+.DELTA.L) which is given by adding the length of the optical fiber Lm, at which the idler light conversion efficiency takes on the first maximal value in the aforementioned periodic function and distance .DELTA.L or 10% of L.