[en] The destruction mechanism of the amplitude oscillation in the system of a cold beam and a weakly collisional plasma is examined numerically. The excitation of the neighboring waves depends not only on the beam strength but also on the beam velocity upsilonsub(b)/upsilonsub(t). Even if s is large, the system becomes stable for the excitation of the neighboring waves with large upsilonsub(b)upsilonsub(t). Consequently, collisional effects become more important for the destruction process because either of the collisional and the neighboring wave effects competes with another in playing a decisive role for the destruction of amplitude oscillations. (author)