1. An endoscope apparatus comprising: an illumination device configured to irradiate at least three kinds of illumination light having different wavelengths including standard light, first reference light, and second reference light onto a biological object serving as a subject; an illumination light switching device configured to periodically switch the at least three kinds of illumination light irradiated by the illumination device in each imaging frame; an imaging device configured to receive return light from the subject onto which the at least three kinds of illumination light switched by the illumination light switching device are irradiated and capture image data in each imaging frame; and a biological information acquisition device configured to acquire biological function information relating to the biological object from the captured image data captured by the imaging device, wherein the illumination light switching device switches an irradiation order of the at least three kinds of illumination light by the illumination device at least in order of the first reference light, the standard light, and the second reference light in the same cycle, and the imaging device acquires at least the image data of a first reference image, the image data of a standard image, and the image data of a second reference image in the same cycle using the respective return light of the at least three kinds of illumination light which are irradiated onto the biological object by the illumination device at least in order of the first reference light, the standard light, and the second reference light in the same cycle, and acquires at least the first reference image, the standard image, and the second reference image, respectively, and the standard light is broadband light including a visible light wavelength band, and the first reference light and the second reference light are narrowband light having a predetermined wavelength bandwidth, and the biological information acquisition device performs a spectral estimation process using captured image data obtained by irradiation of the broadband light, obtains, as the standard image, a first standard wavelength image for comparison with the first reference image captured in the same cycle as the standard image by the first reference light and a second standard wavelength image for comparison with the second reference image captured in the same cycle as the standard image by the second reference light, and calculates the biological function information based on the first reference image and the first standard wavelength image, and/or the second reference image and the second standard wavelength image.