Provided is a data processing device for processing data acquired by an electrophoretic analysis. A graph creator (31, 33) creates an electropherogram and/or a separation image of peaks on an electrophoretic channel. A display processor (37) displays a graph showing the electropherogram and/or the separation image on which a boundary line is drawn to define the inside and the outside of an analysis target range extending along an axis of a separation direction, with the inside and the outside being shown in different visual modes. An analysis condition setter (34) receives an operation by a user for moving the boundary line on the graph displayed and sets the analysis target range whose position and size on the axis are changed according to the received operation. A data-analysis processor (35, 36) carries out a predetermined computation using all or some of peaks included in the analysis target range.