The ferromagnetic phase transition in the invar alloy Fe₇₀Ni₃₀ (0.7 at% C) with fully FCC structure was investigated by means of 3-D neutron depolarization analysis. The mean magnetic induction, magnetic anisotropy parameter and neutron depolarization were measured as a function of temperature and external magnetic field. Our experiments show the appearance of large magnetic inhomogeneities with size about 10³-10⁴ angstroms in the paramagnetic temperature range. The data were interpreted using the concept of a probability function ρ(T_C) which describes the variations of the Curie temperatures over the system.