  Below T_{WFM} ~ 2.75K, a weak ferromagnetism and superconductivity coexist in
ErNi2B2C. In the present paper, a possibility of a spontaneous vortex phase was
examined directly by small angle neutron scattering technique. An external
magnetic field was applied to align ferromagnetic domains so that the internal
magnetic field induced by the weak ferromagnetism becomes uniform and uniaxial
over the bulk sample. When the field is removed, the flux line lattice
subsisted below T_{WFM} but disappeared above it. A pinning effect is also
discussed. We conclude that the spontaneous vortex state is realized in
ErNi2B2C.
