Provided are an image encoding/decoding method and apparatus carrying out in-screen prediction. An image decoding method according to the present invention comprises the steps of: deriving an in-screen prediction mode for the current block; selecting at least one reconstructed sample line adjacent to the current block; establishing a reference sample by means of at least one reconstructed sample included in the at least one reconstructed sample line; and carrying out in-screen prediction for the current block on the basis of the in-screen prediction mode and reference sample.