The effects of filtering velocity it and dust concentration c on pressure drop parameters. i.e., specific resistance of dust cake alpha and specific resistance of filter cloth with residual dust zeta(d), were examined for three kinds of dust in a bag filter. The measured specific resistance alpha for a dust cake with a void fraction larger than 0.5 was almost proportional to the 1/2 power of the filtering velocity and to the -1/3 power of the dust concentration. Comparing the model equation of the filter cloth with dust to the Kozeny-Carman equation, the value. of (1 - epsilon)/epsilon(3) was obtained from the measured value of alpha and correlated empirically with the operating conditions and the physical properties of dust. The resultant correlation gave rise to the following correlation of alpha, alpha = 1.15 x 10(11)(D-v/D-s)(3) D(v)(-3/4)rho(-2/3)c(-1/3)u(1/2)mu(1/2) The estimated values of alpha from the correlation coincided well with the measured ones within the experimental error of +/- 25%. The measured specific resistance of filter cloth with residual dust zeta(d) was almost independent of the filtering velocity and the dust concentration. The dependency of alpha on the variation of the filtering velocity u with time during a filtering operation in a multi-compartment bag filter gave a slightly larger value of the pressure drop than that at the constant value of alpha through the operation.. but did not affect the time-dependent behavior of the pressure drop.