Operation data is obtained from an input device and is stored into a memory, and initial state data indicating an attitude or a position of an object in a virtual game world is calculated with predetermined start timing using acceleration data included in the obtained operation data. Depending on acceleration data included in operation data sequentially obtained on and after the start timing, change amount data for sequentially changing the attitude or position of the object over time is calculated. Thereafter, the attitude or position of the object disposed in the virtual game world, depending on the initial state data, is changed, depending on the change amount data, to perform a game process of controlling a motion of the object.