A method for updating an object detector of an autonomous vehicle to adapt the object detector to a driving circumstance is provided. The method includes steps of: a learning device (a) (i) inputting a training image, corresponding to a driving circumstance, into a circumstance-specific object detector to apply (i-1) convolution to the training image to generate a circumstance-specific feature map, (i-2) ROI pooling to the circumstance-specific feature map to generate a circumstance-specific pooled feature map, and (i-3) fully-connected operation to the circumstance-specific pooled feature map to generate circumstance-specific object detection information and (ii) inputting the circumstance-specific feature map into a circumstance-specific ranking network to (ii-1) apply deconvolution to the circumstance-specific feature map and generate a circumstance-specific segmentation map and (ii-2) generate a circumstance-specific rank score via a circumstance-specific discriminator; and (b) training the circumstance-specific object detector, the circumstance-specific deconvolutional layer, the circumstance-specific convolutional layer, and the circumstance-specific discriminator.