To provide an isopentane fractionation installation for efficiently fractionating isopentane.

This isopentane fractionation method comprises a separation process for distilling desulfurized naphtha subjected to a hydrodesulfurization treatment in a naphtha desulfurization reaction tower 200, in a separation tower 110 to fractionate desulfurized heavy naphtha from the bottom and fractionate the first fractionation component of light fraction from the overhead, a matching process for distilling the first fractionation component in a matching tower 120 connected to the overhead of the separation tower 110 to fractionate a liquidized petroleum gas from the overhead and fractionate the second fractionation compound of liquidized petroleum gas-separated desulfurized naphtha light fraction from the bottom, and a multi-component fractionation process for distilling the second fractionation compound in a division wall distillation tower 130 connected to the bottom of the matching tower 120 to fractionate isopentane and desulfurized medium-gravity naphtha from the overhead and the bottom, respectively, and further side-distill desulfurized light naphtha from the division wall 131. Thus, the isopentane can efficiently be fractionated in a simple structure comprising the required minimum treatment steps.