1. An apparatus for implementing walkthrough display by displaying a plurality of image data held on a network in turn, comprising: a processor for executing a computer-executable program; and a memory medium on which is stored a computer-executable program for implementing a method for implementing the walkthrough display, the method comprising the steps of: acquiring image data of a first landscape from a first server on the network on the basis of geographical relationship data which includes a network identifier required to identify the image data of the first landscape and direction information associated with the direction of a presence of a second landscape with respect to the first landscape; displaying the image data of the first landscape together with pointer information that suggests a geographical relationship of the first landscape to the second landscape based on the geographical relationship data; detecting an input to the pointer information; updating the geographical relationship data when the input to the pointer information is detected; controlling to acquire image data of the second landscape from a second server on the network different from the first server from which the image data of the first landscape was acquired, and to display the image data of the second landscape in substitution for the image data of the first landscape; calculating, when the geographical relationship data associated with the second landscape does not include any network identifier of geographical relationship data associated with another landscape, and the geographical relationship data associated with the first landscape includes direction information associated with the second landscape and direction information associated with a third landscape which has a geographical relationship with the first landscape, direction information to image data of the third landscape with respect to the image data of the second landscape on the basis of the direction information associated with the second landscape and the direction information associated with the third landscape; and generating geographical relationship data associated with the third landscape, which includes the calculated direction information and network identification information required to identify the image data of the third landscape.