1. A stay condition analyzing apparatus that acquires stay information regarding a stay condition of a plurality of moving objects within a target area, generates a heat map image obtained by visualizing the stay information, and displays the heat map image on a display device, the stay condition analyzing apparatus comprising: a processor; a memory which stores an instruction, wherein as a configuration when the processor executes the instruction stored in the memory; a stay information acquirer which acquires, from a plurality of captured images of a same target area, the stay information corresponding to the same target area for each predetermined measurement period of time based on positional information of the moving objects acquired, the plurality of captured images being captured with respect to time; a heat map image generator which generates the heat map image obtained by visualizing the stay information acquired by the stay information acquirer, the heat map image indicating a heat area corresponding to the stay information of the moving objects; a background image generator which selects a target image from the plurality of captured images of the same target area with respect to time, and sets the selected target image as a background image; and a display image generator which generates a display image by superimposing the heat map image generated by the heat map image generator on the background image selected by the background image generator, wherein the background image generator includes a similarity calculator which calculates a similarity of positional information of the moving objects included in each of the plurality of captured images of the same target area and the stay information of the same target area, and a background image selector which selects the target image among the plurality of captured images to be used as the background image based on the similarity calculated by the similarity calculator.