A method of making a two-dimensional measurement of the concentration of a biological macromolecule aqueous solution and an apparatus for carrying out this method are disclosed. By charging the biological macromolecule aqueous solution and an aqueous crystallizing agent solution into a crystallizing cell, irradiating the crystallizing cell with an ultraviolet light from an ultraviolet light source, two-dimensionally scanning the ultraviolet light transmitted through the crystallizing cell by a linear image sensor to detect the quantity of the transmitted light, and carrying out a conversion of the detected quantity of the transmitted light to the protein concentration, the process of a crystallization of the biological macromolecule from biological macromolecule aqueous solution can be easily and accurately twodimensionally measured.