A cohesive powder (Geldart group C) was fluidized under reduced pressure (P = 0.5-10 kPa) with vibration. The fluidized powder was composed of glass beads 6 mum in diameter. The bed pressure drop was measured by decreasing gas velocity and the flow patterns in the bed were observed. A slanting particle flow, which was not observed at atmospheric pressure in a previous study, appeared at a lower pressure than about P = 20 kPa and with a larger vibration strength than the critical vibration strength, Lambda(cr). Under the above conditions, the pressure drop curve changed abnormally due to the occurrence of this slanting particle flow. On the other hand, when the vibration strength was smaller than Lambda(cr), a typical pressure drop curve was obtained. In light of these results, the interrelation between the slanting particle flow and the change in the pressure drop curve was examined.