DESCRIPTION Title of Invention PORTABLE INFORMATION TERMINAL, DISPLAY CONTROL METHOD, AND PROGRAM Technical Field [0001] The present invention relates to a display control method for a portable information terminal that includes a plurality of display screens. Background Art [0002] The portable information terminal represented by a portable telephone, a PDA (Personal Digital Assistant), or an electronic notebook generally includes a display unit that displays a text or an image on the display screen of a liquid crystal or the like. A certain portable information terminal includes a plurality of display screens in its display unit. For example, Patent Literature 1 discloses a portable information terminal that is formed into an openable and closable folding structure having a plurality of cases and that includes a display screen in each of the plurality of cases. The portable information terminal disclosed in Patent Literature 1 is compact and highly portable in a closed state, and can provide a large screen in an opened state. [0003] The portable information terminal of this type may display one image by dividing it to match two display screens. Generally, when the image is divided to be displayed on the two display screens, the image is divided by a dividing line defined to match a size of each display screen. For example, when the two display screens are equal in size, the center line of the original image becomes a dividing line. [0004] Dividing one image into a plurality to display it enables displaying of a character, an icon or a picture included in the image in a magnified manner. However, because of a gap between the display screens, the character, the icon or the picture included in the original image may be divided so that it becomes difficult to see. [0005] To deal with this problem, a technology has been developed which reduces the difficulty of viewing images that is caused by the gap that occurs between the display screen (refer to Patent Literature 2). When one image is divided to be displayed on two display panels, the display device described in Patent Literature 2 shifts the image so as to prevent positioning of the image such that the image will be divided on the character if the character, as a result of the gap that occurs between the display screens when the image is simply divided at the center, becomes divided. Citation List Patent Literature [0006] Patent Literature 1: JP2009-76038A Patent Literature 2: JP2002-006822A Summary of Invention Problems to be Solved by Invention [0007] However, a certain image actually displayed on the display screen includes a plurality of individual images such as characters or pictures, and the arrangement of the individual images are complex in many cases. For example, as a display form, in the case of a sentence that continues over a plurality of rows in which a proportional font is used, the position of the character in the column direction differs from one row to another. In some cases, even when the technology disclosed in Patent Literature 1 is used for such a complex image, the image cannot be shifted to prevent breakage of the characters in all the rows. [0008] The whole image may include, as individual images, not only characters but also images such as pictures. In such a case, the whole image includes various individual images different from one another in size, further complicating the configuration of the whole image. [0009] It is therefore an object of the present invention to provide a technology which resolves the problem in which an image is difficult to view when the image is divided to be displayed on a plurality of display screens in an information processing device that includes a plurality of display screens. Solution to Problems [0010] To achieve the object, a portable information terminal according to the present invention includes: display means including a plurality of display screens; and control means for dividing a whole image including two or more rows of individual images arrayed in a direction where the plurality of display screens are arrayed into a plurality of divided images by a dividing line that passes through gaps between the individual images in the respective rows, and displaying each of the divided images on each of the display screens of the display means. [0011] A display control method in a portable information terminal including a plurality of display screens according to the present invention includes: dividing, by control means, a whole image including two or more rows of individual images arrayed in a direction where the plurality of display screens are arrayed into a plurality of divided images by a dividing line that passes through gaps between the individual images in the respective rows; and displaying, by the control means, each of the divided images on each of the display screens of the display means. [0012] A display control program for causing a computer to execute display control in a portable information terminal including a plurality of display screens according to the present invention includes: a procedure of dividing a whole image including two or more rows of individual images arrayed in a direction where the plurality of display screens are arrayed into a plurality of divided images by a dividing line that passes through gaps between the individual images in the respective rows; and a procedure of displaying each of the divided images on each of the display screens of the display means. Brief Description of Drawings [0013] Fig. 1A shows an example of a whole image including a plurality of individual images. Fig. IB is an explanatory diagram showing dividing line 502 of the whole image shown in Fig. 1A. Fig. 1C shows divided images 503 and 504 divided by the dividing line shown in Fig. IB. Fig. 2 A is a block diagram schematically showing the configuration of a portable information terminal according to this embodiment. Fig. 2B is a block diagram showing a detailed configuration of the portable information terminal according to this embodiment. Fig. 3 is a flowchart showing divided display processing carried out by control unit 100. Fig. 4 is an explanatory diagram showing the coordinate system of the whole image. Fig. 5 is an explanatory diagram showing the calculation of a sum of pixel values of a binary image in an x axis direction in each y-coordinate. Fig. 6A is an explanatory diagram showing processing to set a dividing line in a gap between individual images in each row. Fig. 6B is an explanatory diagram showing processing to set a dividing line in a gap between individual images in each row. Fig. 6C is an explanatory diagram showing processing to set a dividing line in a gap between individual images in each row. Fig. 7 shows an example of a dividing line to divide an individual image in each row. Fig. 8 is an explanatory diagram showing processing when a picture depicting two humans is divided by a dividing line. Fig. 9A shows an example of dividing line 502. Fig. 9B shows divided images 503 and 504 divided by dividing line 502 shown in Fig. 9A. Fig. 10 is a flowchart showing interlocking scroll processing to scroll a screen by a drag operation on a touch panel. Fig. 11A shows the change of a screen display during the drag operation for scrolling. Fig. 11B shows the change of a screen display during the drag operation for scrolling. Fig. 11C shows the change of a screen display during the drag operation for scrolling. Fig. 12A shows the change of a screen display when the drag operation for scrolling ends. Fig. 12B shows the change of a screen display when the drag operation for scrolling ends. Fig. 12C shows the change of a screen display when the drag operation for scrolling ends. Fig. 13 shows the software configuration of the portable information terminal according to this embodiment. Fig. 14 shows the adjustment of the distance between the individual images between divided image 503 and divided image 504. Description of Embodiments [0014] A portable information terminal according to this embodiment includes two display screens, and displays various images on the two display screens according to processing of applications or user's operation inputs. This portable information terminal can divide one image into two to match the two display screens, and display them. [0015] All images to be displayed by the portable information terminal are referred to as a whole image, and the two images acquired by dividing the whole image to be displayed on the two screens are referred to as divided images. Characters or pictures included in the whole image are referred to as individual* images. [0016] Fig. 1A shows an example of the whole image including a plurality of individual images: the example where a plurality of rows of sentences including characters using a proportional font is included in the whole image. Each character corresponds to an individual image, and squares in the drawing indicate individual images. [0017] In the whole image shown in Fig. 1 A, the positions of the characters of a row direction in the respective rows do not match each other between the rows. When the individual images are thus arranged in a complex manner in the whole image, it is difficult to divide the whole image into two by dividing line 501 so as to prevent division of any individual image. [0018] In the portable information terminal according to this embodiment, as shown in Fig. IB, for such a whole image, dividing line 502 is defined to pass through the gap between the characters in each row. The whole image is then divided into two left and right divided images 503 and 504 by dividing line 502. [0019] Specifically, in the portable information terminal according to this embodiment, the rows including the individual images are detected in the whole image, gaps between the individual images are detected for the respective rows, and dividing line 502 is defined to pass through the gaps between the individual images in the respective rows. In the portable information terminal, the whole image is divided by dividing line 502, and divided images 503 and 504 thus acquired are displayed on the respective display screens as shown in Fig. 1C. [0020] According to the portable information terminal of this embodiment, the whole image including the plurality of individual images is divided into two divided images by the dividing line that passes through the gaps between the individual images constituting the rows in a direction where the two display screens are arrayed. This can prevent division of the individual images such as characters or pictures which would be come difficult to see. [0021] Fig. 2 A is a block diagram schematically showing the configuration of the portable information terminal according to this embodiment. Referring to Fig. 2A, the portable information terminal according to this embodiment includes two displays 131 and 132, and control unit 100. [0022] Control unit 100 divides the whole image including the plurality of individual images into two divided images 503 and 503 by dividing line 502 that passes through the gaps between the individual images constituting the rows in the direction where two displays 131 and 132 are arrayed. For example, when displays 131 and 132 are horizontally arrayed, the rows are horizontal. Then, control unit 100 displays two divided images 503 and 504 on respective displays 131 and 132. [0023] The portable information terminal according to this embodiment can interlockingly scroll the whole image divided into two divided images 503 and 504 and displayed on the two display screens. When an operation input is carried out to scroll the whole image, the portable information terminal brings two divided images 503 and 504 close to each other to couple them following the scrolling. After the end of coupling, the portable information terminal scrolls the whole image while maintaining the coupled state. After the end of the operation input for scrolling, the portable information terminal defines new dividing line 502 for the whole image at this time, and displays divided images 503 and 504 acquired by dividing the whole image by dividing line 502 on the respective display screens. [0024] Hereinafter, the portable information terminal according to this embodiment is described in detail. [0025] Fig. 2B is a block diagram showing the detailed configuration of the portable information terminal according to this embodiment. [0026] As shown in Fig. 2B, portable information terminal 10 according to this embodiment includes control unit 100, display control unit 110, input control unit 120, display unit 130, input unit 140, and storage unit .150. [0027] Display unit 130 is a storage device that includes two displays, namely, left display 131 and right display 132, arranged to be adjacent to each other. Left display 131 and right display 132 are equal in size. In the normal use state where left display 131 and right display 132 are horizontally arranged, left display 131 is located on the left side and right display 132 is located on the right side. [0028] Input unit 140 Is a touth-panel input device integrally configured with left display 131 and right display 132 of display unit 130 to detect a user's operation input. However, this is only an example. As other examples, input unit 140 can be a touch panel type not integral with display unit 130 or a push-button type other than the touch panel type. [0029] Display control unit 110 includes left screen display control unit 111 and right screen display control unit 112 respectively corresponding to left display 131 and right display 132. Divided images 503 and 504 are respectively supplied to left screen display control unit 111 and right screen display control unit 112 from control unit 100. Left screen display control unit 111 and right screen display control unit 112 display supplied divided images 503 and 504 on displays 131 and 132. [0030] Input control unit 120 receives the operation input detected by input unit 140, and notifies control unit 100 of the operation input as input information. [0031] Storage unit 150 includes a read-only memory, a random access memory, and a frame buffer. The read-only memory stores various control programs executed by display control unit 110 or fixed data used for processing by display control unit 110. The random access memory temporarily stores data when control unit 100 or display control unit 110 executes a control program or an application program. The frame buffer temporarily stores the whole image displayed on display unit 310. [0032] Control unit 100 controls the respective units in portable information terminal 10 by executing the control programs stored in the read-only memory of storage unit 150. [0033] Further, control unit 100 executes an application based on input information from input control unit 120 or other events. Along with the execution of the application, control unit 100 displays an image on display unit 130. In this case, control unit 100 divides the whole image of the frame buffer into two divided images 503 and 504, and supplies divided images 503 and 504 to left screen display control unit 111 and right screen display control unit 112 of display control unit 110. [0034] When the whole image is divided into two, control unit 100 detects rows including individual images in the whole image, detects gaps between the individual images in the respective rows, and'divides the whole image by dividing line 502 defined to pass through the gaps between the individual images in the respective rows. [0035] Control unit 100 interlockingly scrolls and displays divided images 503 and 504 displayed on the two display screens according to a control input from input control unit 120. [0036] When a scroll operation is input, control unit 100 brings the divided images close to each other to couple them following scrolling of divided images 503 and 504. The dividing line during and after scrolling is a straight line regardless of whether the individual images in the whole image are divided. Then, after the end of the operation input for scrolling, control unit 100 defines new dividing line 502 for the whole image at this time, and displays divided images 503 and 504 acquired by dividing the whole image by dividing line 502 on the respective display screens. [0037] Processing when the image is divided to be displayed on left display 131 and right display 132 is referred to as divided display processing, and processing when the image is scrolled from the state where the image is divided to be displayed is referred to as interlocking scroll processing. [0038] (Divided display processing) First, divided display processing is described. [0039] Fig. 3 is a flowchart showing the divided display processing carried out by control unit 100. Fig. 4 is an explanatory diagram showing the coordinate system of the whole image. [0040] In an example shown in Fig. 4, an x axis is set in the arraying direction of left display 131 and right display 132, and a y axis is set in a direction orthogonal to the x axis. In many cases, image data is stored in the order from left to right and from the upper side to the lower side. Thus, the left upper side is original point O, the x axis is the right direction, and the y axis is the downward direction. In the whole image, the width of the x axis direction is 2W, and the height of the y axis direction is H. [0041] According to this embodiment, dividing line 502 is determined within the range of W-La[m]+b[m])/2 (No in step S210), control unit 100 sets the dividing line at the position of x=(b[m]+a[m+lJ])/2 (step S211). [0054] Next, image division processing to divide the individual image over the width of the divided area is described. This image division processing is for setting a dividing line to divide a large image such as a picture present in the rectangular area of W-LY2 (step S219), searches for the range of the y coordinate where the sum of pixel values in the x axis direction is not 0 for each y axis in the binary image of the divided area shown in a graph on the left side of Fig. 5 (step S204), and repeats a similar series of processes. [0063] When no such range of the y coordinate where the sum of pixel values in the x axis direction is not 0 is detected (No in step S205), control unit 100 completes the processing of setting dividing lines for the respective ranges of the y axis direction, and then connects the dividing lines (step S220). In this processing operation that connects the dividing lines, control unit 100 combines the dividing lines set at intervals in the y axis direction to become continuous lines within the range of 0