A display device that displays an error between an instructed command position of a specific portion of a control target and a response position of the specific portion in a manner easily understandable to a user is described. The display device acquires a response position of the specific portion and calculates a difference in position of the command position and the response position. The display device displays a spatial trajectory based on the command position or a spatial trajectory based on the response position. The display device also displays each spatial trajectory in a mode where a portion of the spatial trajectory of the command position corresponding to the selected mark and a portion of the spatial trajectory of the response position corresponding to the relevant portion of the spatial trajectory of the command position are enlarged at a same magnification.