PURPOSE: To select the amplification band width relatively freely, by providing a part, where the variable constant is changed by excitation, periodically on the surface of an undulation propagating medium and causing a parametric interaction only when the undulation of a certain condition is inputted in the frequency dispersion characteristic.

CONSTITUTION: In the part, where the variable constant is changed, provided on an undulation propagating medium 1, individual parts 2A, 2B, 2C are so operated that the variable constant is changed with respect to time by excitation, and as the whole, the variable constant is changed periodically with respect to position as shown by a waveform 11. On this frequency dispersion characteristic, only when the undulation 11 satisfying a certain condition is inputted, the parametric interaction is caused, and the undulation is propagated through the medium. Thus, the frequency dispersion characteristic of the undulation propagation speed is used positively to design the amplification band relatively freely.