A method for training a main CNN by using a virtual image and a style-transformed real image is provided. And the method includes steps of: (a) a learning device acquiring first training images; and (b) the learning device performing a process of instructing the main CNN to generate first estimated autonomous driving source information, instructing the main CNN to generate first main losses and perform backpropagation by using the first main losses, to thereby learn parameters of the main CNN, and a process of instructing a supporting CNN to generate second training images, instructing the main CNN to generate second estimated autonomous driving source information, instructing the main CNN to generate second main losses and perform backpropagation by using the second main losses, to thereby learn parameters of the main CNN.