An accessory device for use with a game controller (3, 4) that includes at least a first button (43, 65) and a second button (44, 66), the accessory device comprising: a compartment configured to removably secure the game controller therein; a grip portion configured to be held by a user with both hands; a first movable portion; and a second movable portion, the first movable portion including: a first operation portion configured to be pressed by one hand of the user holding the grip portion; and a first button depression portion configured to move toward the first button of the game controller secured in the compartment, thereby pressing the first button of the game controller secured in the compartment, in response to the pressing operation on the first operation portion, the second movable portion including: a second operation portion configured to be pressed by the other hand of the user holding the grip portion; and a second button depression portion configured to move toward the second button of the game controller secured in the compartment, thereby pressing the second button of the game controller secured in the compartment, in response to the pressing operation on the second operation portion, characterized in that: the first movable portion further includes: a first shaft portion; and a first arm portion connecting the first operation portion and the first button depression portion and configured to pivot about the first shaft portion; and the second movable portion further includes: a second shaft portion; and a second arm portion connecting the second operation portion and the second button depression portion and configured to pivot about the second shaft portion. a compartment configured to removably secure the game controller therein; a grip portion configured to be held by a user with both hands; a first movable portion; and a second movable portion, the first movable portion including: a first operation portion configured to be pressed by one hand of the user holding the grip portion; and a first button depression portion configured to move toward the first button of the game controller secured in the compartment, thereby pressing the first button of the game controller secured in the compartment, in response to the pressing operation on the first operation portion, a first operation portion configured to be pressed by one hand of the user holding the grip portion; and a first button depression portion configured to move toward the first button of the game controller secured in the compartment, thereby pressing the first button of the game controller secured in the compartment, in response to the pressing operation on the first operation portion, the second movable portion including: a second operation portion configured to be pressed by the other hand of the user holding the grip portion; and a second button depression portion configured to move toward the second button of the game controller secured in the compartment, thereby pressing the second button of the game controller secured in the compartment, in response to the pressing operation on the second operation portion, characterized in that: a second operation portion configured to be pressed by the other hand of the user holding the grip portion; and a second button depression portion configured to move toward the second button of the game controller secured in the compartment, thereby pressing the second button of the game controller secured in the compartment, in response to the pressing operation on the second operation portion, characterized in that: the first movable portion further includes: a first shaft portion; and a first arm portion connecting the first operation portion and the first button depression portion and configured to pivot about the first shaft portion; and a first shaft portion; and a first arm portion connecting the first operation portion and the first button depression portion and configured to pivot about the first shaft portion; and the second movable portion further includes: a second shaft portion; and a second arm portion connecting the second operation portion and the second button depression portion and configured to pivot about the second shaft portion. a second shaft portion; and a second arm portion connecting the second operation portion and the second button depression portion and configured to pivot about the second shaft portion.