1. A method of decoding a video signal with a decoding apparatus, comprising: deriving reference samples based on neighboring samples adjacent to a current block, the reference samples being used for an intra prediction of the current block; performing the intra prediction on the current block based on the reference samples; deriving a reconstruction sample of the current block using a prediction sample derived by performing the intra prediction and a residual sample relating to the current block; and applying a deblocking filter to the reconstruction sample of the current block, wherein deriving the reference samples comprises: determining, based on a prediction mode of a neighboring block, whether at least one of the neighboring samples included in the neighboring block is an unavailable sample, the prediction mode specifying whether the neighboring block is coded in an inter prediction or the intra prediction; and performing a substitution for the unavailable sample which is not used for the intra prediction of the current block by using a sample located on one side of the unavailable sample among the neighboring samples adjacent to the current block when the unavailable sample is present among the neighboring samples adjacent to the current block, wherein when the unavailable sample corresponds to at least one of left neighboring samples other than a top-left neighboring sample, the substitution of the unavailable sample is performed using only a bottom neighboring sample of the unavailable sample, without referring to a top neighboring sample of the unavailable sample, and wherein when the unavailable sample corresponds to at least one of top neighboring samples, the substitution of the unavailable sample is performed using a left neighboring sample of the unavailable sample.