General electronic devices accept users' operations via mechanical key input units or touch panels. Upon an operation of such an electronic device, each user needs to operate a key input unit or a touch panel while checking an operation position of the key input unit or an indication of the touch panel to reliably operate the electronic device as intended. Hence, in some cases, each user takes a time to operate an electronic device or makes an operation mistake when each user needs to suddenly operate the electronic device or when each user operates the electronic device with the electronic device in a pocket or a bag.
Therefore, operation input methods of electronic devices that make it unnecessary to check an operation position have been proposed (see, for example, Patent Literatures 1 to 3). For example, according to operation input methods disclosed in Patent Literatures 1 and 2, an angular velocity sensor or an acceleration sensor is provided to each electronic device to detect an operation of tapping or shaking each electronic device. Further, according to an operation input method disclosed in Patent Literature 3, a touch screen is provided to an electronic device to detect relative motions of a plurality of fingers at arbitrary positions on the touch screen.    PTL 1: Japanese Patent Application Laid-Open No. 2009-129248.    PTL 2: Japanese Patent Application Laid-Open No. 2013-118700.    PTL 3: Japanese Patent Application Laid-Open No. 2010-108273.
However, according to the operation input methods disclosed in Patent Literatures 1 and 2, it is difficult to distinguish between an acceleration and an angular velocity produced by a user's intentional operation, and an acceleration and an angular velocity produced by a motion of an electronic device which is not intended by a user, and it is difficult to accurately detect a user's operation.
Further, according to the operation input method disclosed in Patent Literature 3, when, for example, a user tries to operate the touch screen while gripping an electronic device by one hand, the user needs to grip the electronic device by an irregular shape of a hand to place a plurality of fingers in contact with the touch screen. Therefore, user's operability for operating the touch screen by a plurality of fingers has been poor.