The second-order magnetic phase transition in the invar Fe₇₀Ni₃₀ alloy was investigated. An intrinsic disorder in invar alloys leads to local variations of the Curie temperature T_C around its average value 〈T_C〉. As a result, large-scale inhomogeneities arise along with the usual critical fluctuations above 〈T_C〉. Three-dimensional ND analysis allows one to obtain T_C, its spread δT_C and the characteristic size R₀ of the areas where T_C-variations occur. According to our experimental findings the `percolative' scenario of the phase transition occurs for this alloy.