A process for inhibiting polymerization of an aromatic vinyl compound during the stage of producing, purifying, storing or transporting the aromatic vinyl compound which not only inhibits an initial polymerization but also inhibits polymerization efficiently over a long period of time and which is excellent in the handleability. The process involves the step of adding a nitrogenous aromatic compound in combination with a sulfonic acid compound to the aromatic vinyl compound during the production, purification, storage of transportation thereof.