To provide a distillation device for thermally cracked oil, performing the fractionation of thermally cracked oil having fluctuation in its composition and properties and enabling medium-gravity oil having the desired flash point to be obtained from the middle stage.

This distillation device 10 has a fractionator 12 fractionating thermally cracked oil, reflux means 16, 18 and 32 returning at least part of medium-gravity oil drawn out of a pulling out tray 24 in a fixed flow rate to the upper part than a distilling mechanism 30 of the fractionator, a cooler 14 cooling at least part of returned medium-gravity oil, a flow rate-regulator 52 regulating both flow rates of the medium-gravity oil cooled by the cooler 14 and the higher temperature medium-gravity oil by-passed the cooler in order to regulate the temperature of returned medium-gravity oil, and a temperature controller 20 controlling the regulator 52 to control the temperature of medium- gravity oil to be returned to the fractionator so as for temperature inside the fractionator near the medium-gravity oil-pulling out stage to become a desired temperature.