A back-and-forth displacement detecting device includes: a sensor that detects an amount of displacement of an information processing device and outputs a detection signal indicating the amount of displacement; and a determination section that determines that, if the amount of displacement indicated by the detection signal exceeds a positive threshold value in one of a former period and a latter period which are time periods of predetermined length before and after a point in time when the detection signal indicates an amount of displacement of zero, the amount of displacement indicated by the detection signal in the other period falls below a negative threshold value, and a sum of the amounts of displacement over both the former and latter periods falls within a predetermined range, the information processing device has performed a displacement of back-and-forth movement from an initial state and has returned to the initial state.