The present invention provides a novel RET inhibitor comprising, as an active ingredient, a compound or a salt thereof that have not been known for their RET inhibitory activity, and also provides an agent for preventing or treating diseases (e.g., malignant tumors) that can be prevented or treated by RET inhibitory activity. The RET inhibitor comprises, as an active ingredient, a compound represented by Formula (I) below or a salts thereof: wherein A, R1 to R3, X, and n are as defined in the specification.