A display device for displaying a change in position of a specific portion of a control target driven by a servo motor (41,42,43) based on a series of command values output by execution of a motion program as a trajectory on a screen of the display device, the display device comprising: a first position acquiring unit for acquiring a series of command positions, which are positions of the specific portion, based on the series of command values when mass and friction of the control target are ignored; a second position acquiring unit (106, 107) acquiring a series of response positions corresponding to the series of command positions, the series of response positions being at least one of a series of positions of the specific portion based on the series of command values when a simulation taking into consideration at least one of the mass and the friction of the control target is performed and a series of positions of the specific portion measured when the control target has been driven based on the series of command values; an error calculating section for calculating a difference in position of the command position and the response position corresponding to the command position; a determination section for determining whether or not the calculated difference is greater than or equal to a predefined threshold value to indicate that an error has occurred; a first display data creating section being configured to create data for displaying on the screen at least one of a spatial trajectory formed by the series of command positions and a spatial trajectory formed by the series of response positions (221, 222, 223), and the first display data creating section being further configured to create data for displaying on the screen a first mark (211, 212, 251, 252, 261, 262, 271, 272) indicating the error has occurred in a portion of the spatial trajectory in which the calculated difference is greater than or equal to the predefined threshold value by configuring the first mark to surround the portion of the displayed spatial trajectory; an accepting unit for accepting a selecting operation of the first mark; a second display data creating section for creating data for displaying each spatial trajectory on the screen in a mode where a portion of a spatial trajectory of the command position corresponding to the selected first mark and a portion of a spatial trajectory of the response position corresponding to the portion of the spatial trajectory of the command position are enlarged at a same magnification, the mode being such that the enlarged portions of the spatial trajectories are overlapping each other; and a display control unit for displaying on the screen data created by the first display data creating section and data created by the second display data creating section. a first position acquiring unit for acquiring a series of command positions, which are positions of the specific portion, based on the series of command values when mass and friction of the control target are ignored; a second position acquiring unit (106, 107) acquiring a series of response positions corresponding to the series of command positions, the series of response positions being at least one of a series of positions of the specific portion based on the series of command values when a simulation taking into consideration at least one of the mass and the friction of the control target is performed and a series of positions of the specific portion measured when the control target has been driven based on the series of command values; an error calculating section for calculating a difference in position of the command position and the response position corresponding to the command position; a determination section for determining whether or not the calculated difference is greater than or equal to a predefined threshold value to indicate that an error has occurred; a first display data creating section being configured to create data for displaying on the screen at least one of a spatial trajectory formed by the series of command positions and a spatial trajectory formed by the series of response positions (221, 222, 223), and the first display data creating section being further configured to create data for displaying on the screen a first mark (211, 212, 251, 252, 261, 262, 271, 272) indicating the error has occurred in a portion of the spatial trajectory in which the calculated difference is greater than or equal to the predefined threshold value by configuring the first mark to surround the portion of the displayed spatial trajectory; an accepting unit for accepting a selecting operation of the first mark; a second display data creating section for creating data for displaying each spatial trajectory on the screen in a mode where a portion of a spatial trajectory of the command position corresponding to the selected first mark and a portion of a spatial trajectory of the response position corresponding to the portion of the spatial trajectory of the command position are enlarged at a same magnification, the mode being such that the enlarged portions of the spatial trajectories are overlapping each other; and a display control unit for displaying on the screen data created by the first display data creating section and data created by the second display data creating section.