An endoscope system, comprising: a light source unit adapted to generate illumination light; an image sensor adapted to image an observation target in a multi-frame observation mode, using the illumination light in a plurality of imaging frames, in which a unit consisting of an accumulation period, which is a maximum period in which electric charges are accumulated in a pixel, and a readout period in which a signal is read out from the pixel is set as a unit of an imaging frame; a light source control unit adapted to change a wavelength band or a spectrum of the illumination light for each imaging frame while continuously turning on an illumination light having a specific wavelength band or spectrum through the accumulation period and the readout period; an imaging control unit adapted to read out the signals from the pixels of the image sensor; an image processing unit adapted to generate one observation image using a plurality of captured images acquired in a plurality of the imaging frames having different wavelength bands or different spectra of the illumination light; a stationary degree calculation unit adapted to calculate a stationary degree_of the observation target using the captured image of a color corresponding to the specific wavelength band or the spectrum commonly acquired in each of the imaging frames, wherein the stationary degree of the observation target is a numerical value indicating the magnitude, the direction, or both the magnitude and direction of movement of the observation target relative to the endoscope; wherein said imaging control unit is adapted to keep constant a total time of the accumulation periods and the readout periods of the plurality of imaging frames for acquiring the plurality of captured images used in generating the one observation image and to extend or to shortens a length of the accumulation period or the readout period of each of the imaging frames in the constant total time, said image processing unit is adapted to use the stationary degree for registration between the plurality of captured images acquired during the plurality of frames. a light source unit adapted to generate illumination light; an image sensor adapted to image an observation target in a multi-frame observation mode, using the illumination light in a plurality of imaging frames, in which a unit consisting of an accumulation period, which is a maximum period in which electric charges are accumulated in a pixel, and a readout period in which a signal is read out from the pixel is set as a unit of an imaging frame; a light source control unit adapted to change a wavelength band or a spectrum of the illumination light for each imaging frame while continuously turning on an illumination light having a specific wavelength band or spectrum through the accumulation period and the readout period; an imaging control unit adapted to read out the signals from the pixels of the image sensor; an image processing unit adapted to generate one observation image using a plurality of captured images acquired in a plurality of the imaging frames having different wavelength bands or different spectra of the illumination light; a stationary degree calculation unit adapted to calculate a stationary degree_of the observation target using the captured image of a color corresponding to the specific wavelength band or the spectrum commonly acquired in each of the imaging frames, wherein the stationary degree of the observation target is a numerical value indicating the magnitude, the direction, or both the magnitude and direction of movement of the observation target relative to the endoscope; wherein said imaging control unit is adapted to keep constant a total time of the accumulation periods and the readout periods of the plurality of imaging frames for acquiring the plurality of captured images used in generating the one observation image and to extend or to shortens a length of the accumulation period or the readout period of each of the imaging frames in the constant total time, said image processing unit is adapted to use the stationary degree for registration between the plurality of captured images acquired during the plurality of frames.