The influence of re-reflection from a wave paddle on irregular incident waves to the model structure is studied theoretically. The theory considers approximately the non-steady propagation onto the clam water.
　The principal results of the present study are as follows:
1) The steady state almost realized at the twice of traveling time by the group velocity of significant wave, when no reflective model exists in the flume.
2) Oscilation in the incident wave spectrum is caused by the influence of the re-reflection, when the reflective model is set up.
3) The spectral energy of incident waves is remarkedly increased and the steady state is not easily achieved, when the reflection from the model is high and no energy dissipation is expected in the flume.
4) The energy dissipation due to the friction along rough flume functions favorably to reduce the influence of re-reflection.