Abstract: An achromatic optical system for beam transformation is used for example with the optical head of an optical disk apparatus. This optical system is arranged in an optical path which is traversed by a beam emitted from a light source to reach a surface to be irradiated. This optical system includes a first prism arranged on the side of the light source and a second prism arranged on the side of the surface to be irradiated. The first and second prisms are constructed in such a manner that each of the prisms has a beam incident angle which is greater than its beam exit angle so as to expand the width of a beam passed therethrough and that with respect to a plurality of transmitted beams of different wavelengths the deviation angle of the beam passed through the optical system on the whole (the angle formed by the incident beam on the first prism and the emerging beam from the second prism) becomes the same for each other.