1. A method for image decoding that supports multiple layers performed by an image decoding apparatus, the method comprising: analyzing a first layer dependency on a current layer based on a video parameter set (VPS) extension; analyzing a second layer dependency on a current slice based on information encoded in a slice unit, wherein analyzing the second layer dependency on the current slice comprises determining whether the current slice uses the first layer dependency of the VPS extension or the second layer dependency of the slice unit, obtaining, in response to a determination that the current slice uses the second layer dependency of the slice unit, first information indicating the number of reference pictures for inter-layer prediction of the current slice and second information identifying reference layers to which the reference pictures belong, the first information and the second information being signaled in the slice unit, and analyzing the second layer dependency on the current slice based on the first information and the second information; and constructing a reference picture list for the current slice based on at least one of the first layer dependency on the current layer and the second layer dependency on the current slice.