1. A method of reading a plurality of film originals, each being mounted with a slide mount, which are placed on an original support of an image reading apparatus in a plurality of orientations, and displaying the plurality of film originals on a monitor unit of a computer connected to the image reading apparatus, the method comprising: an image reading step of: (i) reading each of the plurality of film originals placed on the original support in a mixed manner with a landscape orientation and a portrait orientation, (ii) identifying a number of frames of film originals simultaneously present on the original support, and (iii) cutting out image areas for each of the frames of film originals to generate a plurality of image signals; a placement orientation detection step of detecting a placement orientation for each of the plurality of film originals based on lengths in horizontal and vertical directions of each image signal corresponding to one of the cut out image areas; a display orientation setting step of setting a display orientation for all of the plurality of image signals, by one user selected setting, wherein a user selects from among: (i) a landscape orientation, (ii) an orientation where each of the plurality of image signals is rotated by a predetermined angle regardless of the placement orientation for each of the plurality of film originals detected in said placement orientation detection step, and (iii) an orientation corresponding to the placement orientation for each of the plurality of film originals detected in said placement orientation detection step; an image signal rotation step of executing in accordance with the display orientation set for the plurality of image signals in said display orientation setting step: (i) a first image signal rotation of rotating each of the plurality of image signals to be in the landscape orientation, (ii) a second image signal rotation of rotating each of the plurality of image signals by a predetermined angle irrespective of the placement orientation detected in said placement orientation detection step, or (iii) no image signal rotation; and a read image signal display step of simultaneously displaying the plurality of image signals on one display screen of the monitor unit according to the display orientation set in said display orientation setting step and in a form of a thumbnail type display.