Abstract: An apparatus for measuring the concentration of solids in a flow of water is described. Water in a sampling pipe 2 is divided into two paths by a branch pipe. Water flowing through respective paths then passes through respective filters having different pore sizes. Then, the flow rates of the water passing through each of the filters 9a and 9b is measured by flow meters. An operation circuit successively calculates the relative ratio of the time dependent change of the output from each of the flow meters based on the outputs from each of the flow meters 10a and 10b. The successive ratios are indirectly related to the concentration of solids in the stream large enough to be captured by the smallest pore size filter.