Constant-rate and constant-pressure microfiltration experiments were conducted by using dilute colloids of pool water and BSA solution. The increase in the filtration resistance became marked with increasing filtrate volume per unit membrane area, leading to an increase in the filtration pressure in constant-rate filtration and an increase in the reciprocal filtration rate in constant-pressure filtration. The characteristic filtration equation of constant-rate filtration was also derived based on a consideration of the variation of both the porosity and specific surface area of the membrane caused by the capture of the foulants in the Kozeny-Carman equation, as is the case in constant-pressure filtration. Logarithmic plots based on the characteristic filtration equation were approximated by a linear relationship for each of constant-rate and constant-pressure operations. The slope obtained from constant-rate filtration was roughly in accordance with that obtained from constant-pressure filtration. This implies that the clogging of membrane pores proceeds according to the same mechanism in both cases.