A rigid-scope optical system for an endoscope comprising: an image-formation optical system configured to cause an image in each of wavelength bands to be formed in a predetermined imaging device, the wavelength bands including a fluorescence wavelength band belonging to a near-infrared light wavelength band and a visible light wavelength band; and a color-separation-prism optical system having a dichroic film configured to separate an optical path of light to be imaged by the image-formation optical system into an optical path of the visible light wavelength band and an optical path of the fluorescence wavelength band, wherein the image-formation optical system is configured to cause the respective images to be formed in a fluorescence imaging device and a visible light imaging device, the fluorescence imaging device and the visible light imaging device being disposed to cause an amount of misalignment between a fluorescence image formation position and a visible light image formation position caused by the image-formation optical system to correspond to a difference between an optical path length of fluorescence and an optical path length of visible light, the fluorescence and the visible light forming the respective images via the color-separation-prism optical system, and, characterized in that a focal length of the image-formation optical system is represented by f [mm], and an air-equivalent optical path length from the image-formation optical system to an imaging device is represented by Fb [mm], the image-formation optical system has the focal length and the air-equivalent optical path length that satisfy a condition represented by the following expression, Fb / f > 0.72 and Fb / f < 1.00 wherein the image-formation optical system includes, in order from object side to image side, at least a diaphragm, a first lens group having a positive refractive power, and a second lens group having a positive refractive power, the first lens group including, in order from the object side to the image side, a lens having a negative refractive power with a concave surface facing the object side, and at least one lens having a positive refractive power, the second lens group being a focus group that is configured to perform focusing depending on an object distance, and the image-formation optical system satisfies a condition represented by the following expression, where an air-equivalent optical path length from the image-formation optical system to the color-separation-prism optical system is represented by L [mm], 1.4 < L / f < 1.8 an image-formation optical system configured to cause an image in each of wavelength bands to be formed in a predetermined imaging device, the wavelength bands including a fluorescence wavelength band belonging to a near-infrared light wavelength band and a visible light wavelength band; and a color-separation-prism optical system having a dichroic film configured to separate an optical path of light to be imaged by the image-formation optical system into an optical path of the visible light wavelength band and an optical path of the fluorescence wavelength band, wherein the image-formation optical system is configured to cause the respective images to be formed in a fluorescence imaging device and a visible light imaging device, the fluorescence imaging device and the visible light imaging device being disposed to cause an amount of misalignment between a fluorescence image formation position and a visible light image formation position caused by the image-formation optical system to correspond to a difference between an optical path length of fluorescence and an optical path length of visible light, the fluorescence and the visible light forming the respective images via the color-separation-prism optical system, and, characterized in that a focal length of the image-formation optical system is represented by f [mm], and an air-equivalent optical path length from the image-formation optical system to an imaging device is represented by Fb [mm], the image-formation optical system has the focal length and the air-equivalent optical path length that satisfy a condition represented by the following expression, Fb / f > 0.72 and Fb / f < 1.00 wherein the image-formation optical system includes, in order from object side to image side, at least a diaphragm, a first lens group having a positive refractive power, and a second lens group having a positive refractive power, the first lens group including, in order from the object side to the image side, a lens having a negative refractive power with a concave surface facing the object side, and at least one lens having a positive refractive power, the second lens group being a focus group that is configured to perform focusing depending on an object distance, and at least a diaphragm, a first lens group having a positive refractive power, and a second lens group having a positive refractive power, the first lens group including, in order from the object side to the image side, a lens having a negative refractive power with a concave surface facing the object side, and at least one lens having a positive refractive power, the second lens group being a focus group that is configured to perform focusing depending on an object distance, and the image-formation optical system satisfies a condition represented by the following expression, where an air-equivalent optical path length from the image-formation optical system to the color-separation-prism optical system is represented by L [mm], 1.4 < L / f < 1.8