An optical fiber sensor (1) includes: a setting means (2) that sets an evaluation interval, which is to be set for an optical fiber (11), to either a first interval (L) or a plurality of second intervals (l) which are each shorter than the first interval (L); an extraction means (3) that extracts a change in the condition of a light from the optical fiber (11); and a detection means (4) that detects a change in the surrounding environment on the basis of time series data for the change in the condition of the light during the evaluation interval.