A method for producing a conjugated diolefin that can suppress reaction byproducts with high boiling point from remaining in steps following a quenching step by a quenching tower, and producing a conjugated diolefin (e.g., 1,3-butadiene) that can be used as a raw material for synthetic rubber, resin and the like, even when using an industrial grade mixed hydrocarbon that may not necessarily have a high purity as a raw material to produce the conjugated diolefin (e.g., butadiene). The method includes a step of producing a product gas by reaction containing a conjugated diolefin by feeding a hydrocarbon containing a monoolefin having 4 or more carbon atoms and oxygen to a reactor accommodated with a catalyst containing a metal oxide and a carrier, and a step of sending the product gas by reaction to a quenching tower and washing with a quenching agent (e.g., an organic amine aqueous solution).