A game system according the present embodiment is constituted by including a portable game machine (information processing device) which may be carried by a user, and one or more operation instruments. FIG. 1 is an example non-limiting schematic view illustrating an appearance of a game machine and an operation instrument according to Embodiment 1. The game machine according to the present embodiment is provided with a housing 1 having a substantially-rectangular board-like shape. A display unit 2 is arranged at the substantial center of a front surface 1a of the housing 1. On the housing 1, an analog stick 3a and a cross key 3b are arranged vertically on the left side of the display unit 2. On the housing 1, in a region on the right side of the display unit 2, an attachment unit 4 to which an operation instrument 7 is attached is arranged at the upper side, and four push buttons 3c to 3f are arranged on the lower side. Each of the four push buttons 3c to 3f has a substantially circular shape and is located to correspond to each apex of a diamond. On the housing 1, in a region below the display unit 2, substantially-rectangular three push buttons 3g to 3i are arranged side by side. The game machine according to the present embodiment accepts the user's operation for the analog stick 3a through push button 3i and for the operation instrument 7 attached to the attachment unit 4. The game machine performs information processing concerning a game in accordance with the accepted content of operation.