PURPOSE: To obtain fine image forming characteristics by applying a magnetic field to a projection lens and generating Farady effect at least in a part of lenses constituting the projection lens to change its rfractive index and to control the intensity or direction of the magnetic field.

CONSTITUTION: The lens barrel of the projection lens L consists of a diamagnet ic material or a paramagnetic material and a magnetic field H due to a coil C is also formed on a glass part in the lens L. When monochromatic straight polarized light is advanced in the direction of the magnetic field having intensity H(A/m), the plane of the polarization is rotated by θ(rad) in each length l(m) on the basis of the Farady effect and expressed as θ=VlH by using a Verdet's constant V. The constant V is expressed as V=(θ/CH)Δne by using the angular frequency ω of light, an optical speed C is a vacuum and the change Δne of the refractive index of reverse inner polarization at the absence of the magnetic field, and when ω=2π/λ (λ is the wavelength of light), V=(2π/λH)Δne is formed. Thereby, Δne=(λ/2π)VH is formed and the change of the refractive index is proportional to the intensity H of the magnetic field. Thus, the intensity and direction of the magnetic field can be controlled by changing the refractive index.