A slot die includes a material distribution chamber configured to distribute a casting solution supplied to the material distribution chamber, and a lip in which a slot including an expansion portion is defined and through which the casting solution distributed in the material distribution chamber is discharged, where a width of the expansion portion taken in a first direction is expanded from one side of the expansion portion to a lip end facing the one side and the one side of the expansion portion is apart from the lip end by a predetermined length taken along a second direction which is perpendicular to the first direction and substantially parallel to a flow direction of the casting solution.