A basic study of re-entrainment of dust particles from collecting media of dust collectors by air stream has been made. It is pointed out in the analysis of the re-entrainment mechanism that the bending stress induced by drag plays an important role and that the critical diameter of particle to be re-entrained is proportional to -2/3 power of velocity gradient. It is suggested from experiment that aggregate particles, once collected on the collecting media, are fairly stable in usual operation of dust collectors where velocity gradient is not so large.