PURPOSE: To hold the projection angle of luminous flux projected from the prism optical system nearly constant regardless of variation in the wavelength of the luminous flux by using a compound prism constituted by coupling at least two kinds of prism made of materials having different wavelength dispersion of refractive indexes.

CONSTITUTION: The optical system consists of the compound prism 11 consisting of the 1st prism 11A made of a material with a refractive index n1 having a vertical angle A and the 2nd prism 11B made of a material with a refractive index n2 having a vertical angle B, and the 3rd prism 12 made of a material with a refractive index n3 having a vertical angle C. The 2nd surface of the 1st prism 11A and the 1st surface of the 2nd prism 11B are cemented to each other, and luminous flux is made incident on the 1st surface of the 1st prism at an angle α1, passed through the cemented surfaces, and projected from the 2nd surface of the 2nd prism; and the luminous flux is incident on the 1st surface of the 3rd prism arranged having its 1st surface at an angle D to the 2nd surface of the 2nd prism and projected from the 2nd surface at an angle 2. When the respective angles satisfy the specific relation of variation in refractive index, the projection angle 2 is constant even if the proejction angle 2 varies.