The three-dimensional separation in a decelerating channel flow generated by the suction through a porous wall had been investigated experimentally. Unsteady characteristics of two corner separations were especially focused on. The value of cross-correlation function gets large negative values without any lag and the value of coherence gets peak value at around 0.5-2Hz. The corner separations near the top wall and near the bottom wall are not independent and keep coherent structure both with negative value of cross correlation function and with the frequency of around 0.5-2Hz.