The present invention provides a polarizing element which has an excellent antireflection function as being applied to an image display device; and a circularly polarizing plate and an image display device, each of which has a polarizing element. The polarizing element has an alignment film and an anisotropic light-absorbing film formed using a dichroic substance, in which a degree S of alignment of the anisotropic light-absorbing film is 0.92 or more, an average refractive index nave at a wavelength of 400 to 700 nm of the alignment film is 1.55 or more and less than 1.78, an in-plane refractive index anisotropy Δn at a wavelength of 550 nm of the alignment film is less than 0.10.