A control section performs first preliminary imaging in which an observation target is illuminated with first blue light at a wavelength band of 450±10 nm and an image of the observation target is captured. A yellow pigment concentration calculator calculates concentration of the yellow pigment based on an image signal obtained by performing the first preliminary imaging. A prescribed exposure amount calculator calculates a prescribed exposure amount of second blue light at a wavelength band of 470±10 nm for second preliminary imaging based on the calculated concentration of the yellow pigment. A control section performs second preliminary imaging in which an observation target is illuminated with the second blue light and an image of the observation target is captured based on a prescribed exposure amount of the second blue light for the second preliminary imaging. An arithmetic value calculator calculates an arithmetic value based on an image signal obtained by performing the second preliminary imaging. A determination section determines whether or not the arithmetic value has become closer to an optimum value.