In a second vessel pattern imaging mode, a first laser beam having a wavelength of 445 nm and a second laser beam having a wavelength of 405 nm are emitted at a second intensity ratio at which the light intensity of the second laser beam is higher than that of the first laser beam. An isosbestic wavelength of oxyhemoglobin and deoxyhemoglobin exists between the wavelengths 405 nm and 445 nm. An electronic endoscope (11) takes an image of an internal body portion irradiated with illumination light, including the first and second laser beams and fluorescence, and outputs an image signal. When vessel contrast, being contrast between a vessel area and a mucosa area obtained from the image signal, does not coincide with a standard value, the second intensity ratio and a color tone of the mucosa area are adjusted to produce a vessel pattern image having the standard vessel contrast.
