The purpose of the present invention is to provide a novel RET inhibitor and a prophylactic or therapeutic agent for diseases that can be prevented or treated with the assistance of a RET inhibition activity (e.g., malignant tumors), each of which contains, as an active ingredient, a compound that is not known to have a RET inhibition activity so far or a salt of the compound. A RET inhibitor containing, as an active ingredient, a compound represented by formula (I) [wherein A, R1 to R3, X and n are as defined in the description] or a salt thereof.
