An endoscope system comprising: a light source for emitting first light at a first wavelength band in which a light absorption amount changes in accordance with a concentration of yellow pigment contained in an observation target, and second light at a second wavelength band in which a light absorption amount changes in accordance with an oxygen saturation level of hemoglobin contained in the observation target and the light absorption amount also changes in accordance with the concentration of the yellow pigment, the second wavelength band being longer than the first wavelength band; an image capturing sensor for capturing an image of the observation target; and a control section for controlling at least one of the light source and the image capturing sensor, so as to perform first preliminary imaging for imaging the observation target illuminated with the first light for the first preliminary imaging and perform second preliminary imaging for imaging the observation target illuminated with the second light for the second preliminary imaging; characterized by a yellow pigment concentration calculator for calculating the concentration of the yellow pigment based on a first image signal obtained by performing the first preliminary imaging; a prescribed exposure amount calculator for calculating a prescribed exposure amount of the second light in the second preliminary imaging based on the calculated concentration of the yellow pigment; an arithmetic value calculator for performing a specific calculation based on a second image signal obtained by performing the second preliminary imaging in accordance with the prescribed exposure amount of the second light, so as to calculate an arithmetic value; and a determination section for determining whether or not the arithmetic value has become closer to an optimum value specified in advance. a light source for emitting first light at a first wavelength band in which a light absorption amount changes in accordance with a concentration of yellow pigment contained in an observation target, and second light at a second wavelength band in which a light absorption amount changes in accordance with an oxygen saturation level of hemoglobin contained in the observation target and the light absorption amount also changes in accordance with the concentration of the yellow pigment, the second wavelength band being longer than the first wavelength band; an image capturing sensor for capturing an image of the observation target; and a control section for controlling at least one of the light source and the image capturing sensor, so as to perform first preliminary imaging for imaging the observation target illuminated with the first light for the first preliminary imaging and perform second preliminary imaging for imaging the observation target illuminated with the second light for the second preliminary imaging; characterized by a yellow pigment concentration calculator for calculating the concentration of the yellow pigment based on a first image signal obtained by performing the first preliminary imaging; a prescribed exposure amount calculator for calculating a prescribed exposure amount of the second light in the second preliminary imaging based on the calculated concentration of the yellow pigment; an arithmetic value calculator for performing a specific calculation based on a second image signal obtained by performing the second preliminary imaging in accordance with the prescribed exposure amount of the second light, so as to calculate an arithmetic value; and a determination section for determining whether or not the arithmetic value has become closer to an optimum value specified in advance.