The radial distribution of solid concentration in the full-developed flow region of a slurry bubble column was measured by an ultrasonic penetration method. The method allows the measurement of the solid holdup separately from the gas holdup hg measuring the transmission time of the ultrasound transmitted through the three-phase medium. The solid concentration is maximum in the region near the column wall. The peak is sharpened with an increase in either the gas superficial velocity or the particle diameter, and is flattened with an increase in the liquid superficial velocity. The mechanism of the particle distribution is discussed by considering the role of lift force, diffusion force, interaction force caused by impaction between particles and bubbles, and interaction between the particles and liquid vortices in the bubble wakes. The effect of the interaction force seems to be significant in determining the structure of the particle concentration profiles in the slurry bubble column.