It has been shown that, in a nonlinear vibratory system subjected to several harmonic excitations of frequencies Ω₁, Ω₂, …, Ω_M, so-called combination tone can be induced with frequency Ω=|m₁Ω₁+m₂Ω₂+…+m_MΩ_M| (m₁, m₂, …=±1, ±2, …), When Ω is close to the natural frequency of the system. Also in this system a more general type of oscillation can be expected to occur with frequency Ω=(1/N)|m₁Ω₁Ω₂+…+m_MΩ_M| (N=2, 3, 4, …; m₁, m₂, …=±1, ±2, …) When Ω is close to the natural frequency. Such oscillations, if they occur, may be termed "sub-combination tone." The present paper concerns the occurrence of such oscillations in a typical case in which a system with nonlinear spring characteristics of a cubic function of the displacement is subjected to two periodic forces of frequencies Ω₁ and Ω₂. The theoretical analysis shows that the sub-combination tones of frequencies Ω=(1/2) |Ω₁±Ω₂| can occur in the system. The theoretical analysis is checked by an analog-computer.