A method for controlling an image processing device that receives a plurality of individual images including at least two images that form a stereoscopic image and generates a display image displaying the plurality of input individual images simultaneously on a display unit, the controlling method comprising:
- an image conversion step of converting at least one image among the input images that form the stereoscopic image into a planar image;
- and
- an image generation step of generating the display image by synthesizing the planar image that has been converted in the image conversion step and an image among the plurality of input images that has not been converted into a planar image in the image conversion step.