Periodic perturbations of on oscillator x¨+x 3 +axx˙ = 0, a 2 < 8, are considered. Smallness of perturbations is defined by the smallness of the neighborhood of the state of equilibrium x = 0 and by a small positive parameter. Branching of the state of equilibrium x = 0 to invariant two-dimentional tori is under consideration. Refs 4.