An image decoding method according to the present document may comprise: a step for deriving an inter prediction mode from encoded information; a step for constructing reference pictures lists; a step for deriving motion information including reference picture indices for symmetric motion vector differences (SMVD) on the basis of reference pictures included in the reference picture lists; and a step for generating prediction samples on the basis of the motion information. The reference picture indices for SMVD may be derived on the basis of short-term reference pictures included in the reference picture lists.