Transient behavior of the bed collapse after shut-off the gas supply into a three-phase fluidized bed was determined. Experiments were carried out in a 210-mm diameter half-tube acrylic column having a 1.8 m-high test section. The polymer beads (d_p=3.2 mm, r_s=1,280 kg/m³) were fluidized by cocurrent flow of deionized water and air. The transient behavior of the bed collapse after cut-off the gas supply to the bed was monitored by a video camera (30 frames/s). The dense bed surface height was measured from the image of videotape. At lower liquid velocity, the dense bed surface increases with the elapsed time and then reaches a bed height, whereas at higher liquid velocity the dense bed surface increases sharply with the elapsed time, then decreases and reaches the bed height corresponding to the liquid-solid fluidized beds (water-polymer beads).