A semiconductor device including an input unit to which an acceleration signal representing acceleration is input from an acceleration detecting unit installed in a portable terminal device, and a detection unit which detects that a tap operation is performed on the portable terminal device, in a case in which the acceleration signal input to the input unit reaches both of a first threshold value set on the positive side and a second threshold value set on the negative side and a period until the acceleration signal exceeds any one of the first threshold value and the second threshold value and then exceeds the other threshold value is equal to or more than a predetermined first period and is equal to or less than a second period larger than the first period.