A game system comprising: an input device including an acceleration sensor detecting linear accelerations in three axial directions consisting of a left/right-direction of the input device, an up/down-direction of the input device and a forward/backward direction of the input device; acquisition means (10, S2) for sequentially acquiring acceleration data sequentially outputted by the acceleration sensor; smoothing means (10, S3) for sequentially calculating a smoothed vector representing a vector which is obtained by smoothing an acceleration vector represented by the acceleration data in the left/right direction of the input device and the up/down direction of the input device, the smoothing means calculating the smoothed vector each time the acquisition means acquires the acceleration data, and performing a calculation such that the smoothed vector having been most recently calculated approaches, at a predetermined rate, an acceleration vector represented by the acceleration data having been acquired by the acquisition means; and game process means (10, S4) for executing a game process using the acceleration vector and the smoothed vector, the game process means including first action control means (10, S11) for calculating, using the smoothed vector, an attitude of a player object in a game space based on a tilt of the input device corresponding to an amount of rotation about the axis in forward/backward direction of the input device, to control the attitude of the player object in the game space in accordance with the tilt of the input device, and second action control means (10, S12-S17) for calculating, using the acceleration vector, a position of the player object based on a movement of the input device in the up/down-direction of the input device, to control the position of the player object in accordance with the movement of the input device. an input device including an acceleration sensor detecting linear accelerations in three axial directions consisting of a left/right-direction of the input device, an up/down-direction of the input device and a forward/backward direction of the input device; acquisition means (10, S2) for sequentially acquiring acceleration data sequentially outputted by the acceleration sensor; smoothing means (10, S3) for sequentially calculating a smoothed vector representing a vector which is obtained by smoothing an acceleration vector represented by the acceleration data in the left/right direction of the input device and the up/down direction of the input device, the smoothing means calculating the smoothed vector each time the acquisition means acquires the acceleration data, and performing a calculation such that the smoothed vector having been most recently calculated approaches, at a predetermined rate, an acceleration vector represented by the acceleration data having been acquired by the acquisition means; and game process means (10, S4) for executing a game process using the acceleration vector and the smoothed vector, the game process means including first action control means (10, S11) for calculating, using the smoothed vector, an attitude of a player object in a game space based on a tilt of the input device corresponding to an amount of rotation about the axis in forward/backward direction of the input device, to control the attitude of the player object in the game space in accordance with the tilt of the input device, and second action control means (10, S12-S17) for calculating, using the acceleration vector, a position of the player object based on a movement of the input device in the up/down-direction of the input device, to control the position of the player object in accordance with the movement of the input device.