The present invention provides a novel RET inhibitor comprising, as an active ingredient, a compound or a salt thereof, 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) or a salts thereof: wherein A, X, and R2 are as defined in the specification.