We researched the AIC (Alfvén Ion Cyclotron) instability. A magnetic probe using microchip-inductor is superior to size and easy to handle. In fluctuation measurement using the GNBP (Gold Neutral Beam Probe) the measured fluctuations may not be local density fluctuation because the beam ionization coefficient depends on not only the electron density but also the temperature, and the secondary beam is deflected by the magnetic field fluctuation, and the beam fluctuation is integrated along the beam trajectory. But we found by simulations such as the AIC fluctuation with small amplitude and high frequency these effects may be neglected. In initial experiment, the GNBP detected successfully the AIC fluctuations with high frequencies excited in the core plasma.