A display device for displaying on a screen an operation of a control target device which is operated by an input of a command value of a position outputted by execution of a control program in a predefined period, as a moving image of a virtual mechanism (1800, 1900, 2000, 2100, 2200, 2100, 2200, 2300, 2400, 2500), the virtual mechanism being an electronically constructed three-dimensional model, the display device being a computer storing the control program and connectable to a programmable controller controlling the control target device, the device comprising: a simulation section configured to execute the control program to function as a simulator of the programmable controller and calculate command values of a sequence of positions; a transmitting section configured to transmit a copy of the control program to the programmable controller; a virtual mechanism presenting section configured for presenting options of a virtual mechanism on the screen; a virtual mechanism accepting section configured for accepting one selection of the virtual mechanism out of the options of the virtual mechanism presented by the virtual mechanism presenting section; an acquirement section configured for acquiring the command values of the sequence of positions as execution results of the control program or actual measured values of a sequence of positions from the programmable controller; a moving image data creating section configured for creating moving image data showing a state of the selected virtual mechanism in every period or in a plurality of predefined periods by use of the acquired command values of the sequence of positions or the actual measured values of the sequence of positions, the moving image data being created based on trajectory data showing a trajectory that passes through each of the sequence of positions; and a display controlling section configured for displaying a moving image on the screen by use of the moving image data, wherein the virtual mechanism includes a structure in which a plurality of mechanism elements are combined having interlocking relationships, the virtual mechanism presenting section presents at least options of a virtual mechanism having a linear moving element as a mechanism element, the linear moving element has a columnar linear moving shaft and a movable section that moves along the linear moving shaft, the moving image data creating section determines a length of the linear moving shaft by checking a range of the command values of the sequence of positions or a range of the actual measured values of the sequence of positions, and the linear moving element is displayed in a mode where the length of the linear moving shaft remains unchanged from the beginning to the end of reproduction of at least one moving image. a simulation section configured to execute the control program to function as a simulator of the programmable controller and calculate command values of a sequence of positions; a transmitting section configured to transmit a copy of the control program to the programmable controller; a virtual mechanism presenting section configured for presenting options of a virtual mechanism on the screen; a virtual mechanism accepting section configured for accepting one selection of the virtual mechanism out of the options of the virtual mechanism presented by the virtual mechanism presenting section; an acquirement section configured for acquiring the command values of the sequence of positions as execution results of the control program or actual measured values of a sequence of positions from the programmable controller; a moving image data creating section configured for creating moving image data showing a state of the selected virtual mechanism in every period or in a plurality of predefined periods by use of the acquired command values of the sequence of positions or the actual measured values of the sequence of positions, the moving image data being created based on trajectory data showing a trajectory that passes through each of the sequence of positions; and a display controlling section configured for displaying a moving image on the screen by use of the moving image data,