An interference spectrophotometer including a movable mirror unit having a movable mirror capable of reciprocating movement; a stationary mirror; an infrared light source unit which emits an infrared light; a beam splitter; an interference light detection unit which detects light intensity information of light transmitted or reflected by a sample; a movable mirror velocity information detection unit which detects movable mirror velocity information for movable mirror; and a control unit which acquires the light intensity information and movable mirror velocity information and computes the absorption or transmission spectrum of the sample; wherein the interference spectrophotometer further comprises a storage unit which stores a target movable mirror velocity range, and control unit does not employ light intensity information obtained when the movable mirror velocity of movable mirror was outside the target movable mirror velocity range for computing the absorption or transmission spectrum of the sample.