We have prepared optically birefringence materials consisting of an isotropic core of metal nanoparticle and an anisotropic shell of amorphous oxide. The sample shows an enhanced optical birefringence in a wavelength-selective way. The sample was prepared by depositing amorphous iron oxide thin films on top of the silver nanoparticles using the oblique deposition technique. This results in ellipsoidal shell of amorphous iron oxide surrounding a silver nanoparticle. The form birefringence appears because of the anisotropic shape of shells; the refractive index for the light polarized parallel to the elongation direction of ellipsoid is different from that for the light polarized perpendicularly. Moreover, the rotation of polarization plane is significantly enhanced at around the wavelength of localized surface plasmon resonance (LSPR). The difference in refractive index between two optical axes is as large as 0.34 for a 600 nm light, which is more than twice of typical birefringence crystal calcite (0.14 for visible light). It is speculated that the anisotropic shell induces the dependence of LSPR wavelength on the polarization direction of the incident light, which causes the polarization dependence of refractive index through the Kramers-Kronig relation.