A fast X-ray CT scanner, designed for measurement of dynamic events of multiphase fluids, generated bubble distribution images at 0.6 m above a gas distributor for a fluidized catalyst bed. The column used for the measurement was an acrylic resin tube of 46 mm in diameter 1.5 m in height. The bed was fluidized by air at superficial gas velocities of 0.01 to 0.07 m s(-1). We focused our analysis on the interface boundary around a rising bubble. We investigated the voidage change in the boundary. The interface-area of a sphere whose diameter is equivalent to the vertical length of a bubble was compared with that of a pseudo-3 D bubble shape obtained by the fast X-ray CT scanner. As a result, we found that the interface area of the deformed bubble was much larger than that of the spherical one.