A control unit of a medical device acquires a plurality of OCT signals, at different times, detected using reflected light of measurement light irradiated toward a measurement section of a biological tissue (S3). By processing the plurality of OCT signals, the control unit calculates a rate of change over time of a local optical path length of the measurement light (S4 to S6). The control unit calculates, on the basis of the rate of change over time of the local optical path length, a cumulative deformation amount obtained through accumulation in a time direction of a deformation amount (≥ 0) of the biological tissue in the measurement section (S7 to S9). As a result, a change in the biological tissue caused by irradiation of treatment light is measured.
