The present disclosure provides a method for producing an ionic liquid, the method comprising: reacting a nitrogen-containing heterocyclic compound or an amine-based compound with an ammonium salt along with trialkyl orthoformate to acquire an alkylated nitrogen-containing heterocyclic compound or an alkylated nitrogen-containing amine-based compound, wherein the alkylated nitrogen-containing heterocyclic compound or the alkylated nitrogen-containing amine-based compound as a cation of the ionic liquid is ionically bonded to an anion included in the ammonium salt to form the ionic liquid.