An in vivo monitoring method in a laparoscope system (2) is provided. An object image (92) is sequentially created with expression of a surface color of an object in a body cavity. A lock area (specific area) (98) is determined within the object image, the lock area being movable by following motion of the object. A monitor image (93, 94) including a graph of oxygen saturation is generated according to a part image included in the object image and located in the lock area. The monitor image is displayed. Preferably, the oxygen saturation of the lock area is acquired according to two spectral data with respect to wavelengths of which an absorption coefficient is different between oxidized hemoglobin and reduced hemoglobin in data of the object image. The object is constituted by a blood vessel.
