A high-frequency (∼0.4 MHz) fluctuation is excited by an electron temperature gradient (ETG) perpendicular to magnetic field lines, which is consistent with an ETG mode. When the fluctuation amplitude of the ETG mode exceeds a certain threshold, the mode gradually becomes saturated and a low-frequency (∼7 kHz) fluctuation which is originally caused by a drift wave is enhanced, corresponding to the saturation of the ETG mode. In addition, a nonlinear coupling, specifically, the bicoherence between the ETG mode and the drift wave mode, begins to increase when the ETG strength exceeds the threshold, which simultaneously occurs with the saturation of the ETG mode. Thus, it was determined that the ETG mode stimulates the drift wave mode excitement via multiscale nonlinear interaction between the high-frequency (∼MHz) and low-frequency (∼kHz) fluctuations, which ultimately causes ETG mode energy to be transferred to the drift wave mode.