  The low-lying spectrum of the three-dimensional Ising model is investigated
numerically; we made use of an equivalence between the excitation gap and the
reciprocal correlation length. In the broken-symmetry phase, the magnetic
excitations are attractive, forming a bound state with an excitation gap
m_2(<2m_1) (m_1: elementary excitation gap). It is expected that the ratio
m_2/m_1 is a universal constant in the vicinity of the critical point. In order
to estimate m_2/m_1, we perform the numerical diagonalization for finite
clusters with N \le 15 spins. In order to reduce the finite-size errors, we
incorporated the extended (next-nearest-neighbor and four-spin) interactions.
As a result, we estimate the mass-gap ratio as m_2/m_1=1.84(3).
