A method for fabricating a prism that comprises a first and a second substrate, both translucent, bonded together with an optical thin film interposed at the interface in between and that is used with the interface inclined relative to the optical axis of incident laser light of a wavelength of 420 nm or shorter includes the step of bonding together the first and second substrates of which the difference .DELTA.N1 in refractive index at the wavelength of the laser light fulfills the condition: .DELTA.N1.ltoreq.|1/(0.3.times.10.sup.4.times.NA.times.t)|, where t represents the thickness of the first and second substrates cemented together as measured along the optical axis of the laser light, and NA represents the numerical aperture of the incident laser light.