In order to measure a transit time of oxygen in blood, at least one sensor is attached to at least one prescribed position on a subject. The sensor is adapted to detect a light absorption of arterial blood at the prescribed position. An oxygen saturation of the arterial blood at the prescribed position is calculated based on the detected light absorption. An amount of oxygen inspired into the subject is varied at a reference time point. A change in the calculated oxygen saturation at the prescribed position is detected. A time period from the reference time point to a time point at which the oxygen saturation changes is measured.