A post-heating treatment device includes a detecting device for a height variation of a surface of the rail at every, predetermined pitch along a length direction of the rail, a control unit for defining a position of the rail to be a starting point of the welded section when the height variation detected by the detecting device at every predetermined pitch exceeds a predetermined threshold for the number of a predetermined times consecutively and defining a position of the rail to be an end point of the welded section when the height variation detected by the detecting device at every predetermined pitch is below the predetermined threshold for the number of the predetermined times consecutively, and a heating unit for heat treatment based on a position of the welded section detected by the detecting device. A post-heating treatment method using the post-heating treatment device is provided.