Experimental study on the optical input-output characteristics of a hole formed with magnetic fluid and thin annular permanent magnet in the alternating magnetic fields were performed by using the photodiode sensor system. The hole of magnetic fluid was driven by the Helmholtz coil. The expansion and contraction motions of magnetic fluid hole in high frequency region was measured with the photodiode sensor system. The light intensity through the magnetic fluid hole was measured by the photodiode sensor system. The output signal was analyzed by a FFT analyzer. The optical input-output characteristics of the magnetic fluid hole in alternating fields were revealed.