A method for learning a runtime input transformation of real images into virtual images by using a cycle GAN capable of being applied to domain adaptation is provided. The method can be also performed in virtual driving environments. The method includes steps of: (a) (i) instructing first transformer to transform a first image to second image, (ii-1) instructing first discriminator to generate a 1_1-st result, and (ii-2) instructing second transformer to transform the second image to third image, whose characteristics are same as or similar to those of the real images; (b) (i) instructing the second transformer to transform a fourth image to fifth image, (ii-1) instructing second discriminator to generate a 2_1-st result, and (ii-2) instructing the first transformer to transform the fifth image to sixth image; (c) calculating losses. By the method, a gap between virtuality and reality can be reduced, and annotation costs can be reduced.