An image display device comprising: a memory configured to store an image file including a main image and a thumbnail image (101, 201) of a standard size, wherein the thumbnail image includes an effective image area corresponding to a reduced version of the main image maintaining its aspect ratio, wherein a pair of blanks indicating margin areas outside of the effective image area of the thumbnail image are formed along a pair of opposite sides of the effective image area of the thumbnail image in either a horizontal direction or a vertical direction, and a display configured to collectively display a plurality of display images on a screen said image display device is characterized by comprising: an image file extraction part configured to read the image file from the memory so as to extract the thumbnail image included in the image file; and a display image generation part configured to generate a display image (103, 203) having a square shape of a predetermined size based on the effective image area of the thumbnail image according to (i) through (iv), (i) specifying a short-side length of the effective image area of the thumbnail image having a length of a shorter one of a horizontal length and a vertical length of the effective image area along the horizontal direction and the vertical direction in the thumbnail image; (ii) producing a resized image (102, 202) by resizing the effective image area of the thumbnail image such that the short-side length of the effective image area corresponds to a one-side length of a square shape which is determined in advance; (iii) detecting a center (P1, P2) of the resized image; and (iv) extracting a center portion of the resized image having the square shape about the center of the resized image as the display image. a memory configured to store an image file including a main image and a thumbnail image (101, 201) of a standard size, wherein the thumbnail image includes an effective image area corresponding to a reduced version of the main image maintaining its aspect ratio, wherein a pair of blanks indicating margin areas outside of the effective image area of the thumbnail image are formed along a pair of opposite sides of the effective image area of the thumbnail image in either a horizontal direction or a vertical direction, and a display configured to collectively display a plurality of display images on a screen said image display device is characterized by comprising: an image file extraction part configured to read the image file from the memory so as to extract the thumbnail image included in the image file; and a display image generation part configured to generate a display image (103, 203) having a square shape of a predetermined size based on the effective image area of the thumbnail image according to (i) through (iv), (i) specifying a short-side length of the effective image area of the thumbnail image having a length of a shorter one of a horizontal length and a vertical length of the effective image area along the horizontal direction and the vertical direction in the thumbnail image; (ii) producing a resized image (102, 202) by resizing the effective image area of the thumbnail image such that the short-side length of the effective image area corresponds to a one-side length of a square shape which is determined in advance; (iii) detecting a center (P1, P2) of the resized image; and (iv) extracting a center portion of the resized image having the square shape about the center of the resized image as the display image. an image file extraction part configured to read the image file from the memory so as to extract the thumbnail image included in the image file; and a display image generation part configured to generate a display image (103, 203) having a square shape of a predetermined size based on the effective image area of the thumbnail image according to (i) through (iv), (i) specifying a short-side length of the effective image area of the thumbnail image having a length of a shorter one of a horizontal length and a vertical length of the effective image area along the horizontal direction and the vertical direction in the thumbnail image; (ii) producing a resized image (102, 202) by resizing the effective image area of the thumbnail image such that the short-side length of the effective image area corresponds to a one-side length of a square shape which is determined in advance; (iii) detecting a center (P1, P2) of the resized image; and (iv) extracting a center portion of the resized image having the square shape about the center of the resized image as the display image. (i) specifying a short-side length of the effective image area of the thumbnail image having a length of a shorter one of a horizontal length and a vertical length of the effective image area along the horizontal direction and the vertical direction in the thumbnail image; (ii) producing a resized image (102, 202) by resizing the effective image area of the thumbnail image such that the short-side length of the effective image area corresponds to a one-side length of a square shape which is determined in advance; (iii) detecting a center (P1, P2) of the resized image; and (iv) extracting a center portion of the resized image having the square shape about the center of the resized image as the display image.