Abstract: A method for measuring a biomass which comprises measuring an electric capacitance across at least one pair of electrodes attached to a bioreactor, and thereby continuously measuring a biomass of organisms (such as microorganism and plant or animal cells), which may or may not be immobilized in the bioreactor, according to the electric capacitance (dielectric permitivity) measured. The present invention permits one to measure on-line the quantities of microorgansisms or plant or animal cells without having to take samples from a bioreactor or culture tank.