PURPOSE: To continuously measure the concentration of solid components in an aqueous solution or a slurry liquid containing a high concentration of suspension by passing the aqueous solution containing the suspension continuously through a passage cell to measure the electric conductivity thereof continuously.

CONSTITUTION: In an absorbing liquid of a lime process wet type exhaust gas desulfurizer, changes in the concentration of solid components during the circulation thereof are detected to perform a process control. A part of the absorbing liquid is introduced into a passage type cell 3 through a valve 1 and a pump 2 and passed between platinum electrodes 4 arranged in parallel to measure the specific resistance (electric conductivity) of the absorbing liquid with a vibration capacity potential difference meter 6. An ultrasonic wave generation terminal is provided at a cell inlet 7 and gives the absorbing liquid an ultrasonic wave to agitate. This helps uniform dispersion of slurry to reduce the scatter in the measured value of the electric conductivity even when the passage rate of the absorbing liquid is relatively slow. Thus, the concentration of a suspension can be measured continuously even in the case of the suspension with a high concentration.