To provide an external storage medium which enables a game to meet the variety of needs of players and to provide a portable game machine to allow players to perform a game by mounting the medium to the machine.

The portable game machine includes a display part to display images of characters or the like appearing in the game, an operation part which is operated by a player, while the player is looking at the images displayed on the display part, a case body to accommodate the operation part and display part, and an acceleration sensor to sense the motion of the case body when the player carries the case body by hand to move it by hand. The external storage medium can be mounted to the game machine, and in the medium, data relating to images displayed on the display part are stored, and a program is stored in the medium so that when images on the display part are deformed, the player moves the case body to correct the deformation of the image on the display part, depending on the relationship between the movement of the image on the display part and the sensing result of the acceleration sensor.