An image decoding method and device according to the present disclosure may determine a transform type for inverse transform of a current block from one or more MTS candidates available to the current block, and perform inverse transform on transform coefficients of the current block on the basis of the determined transform type, so as to acquire residual samples of the current block. Here, the number of the one or more MTS candidates available to the current block may be determined on the basis of at least one among a sum of the transform coefficients in the current block, the number of the transform coefficients in the current block, sizes of the one or more transform coefficients in the current block, a location of a last significant coefficient in the current block, and the number of one or more non-zero coefficients in the current block.