The present invention provides a measuring method most suitable for measuring the concentrations of specific constituents, especially urine protein level and sugar level. After a protein-contained liquid sample is opacified by heating or while the sample is being heated, a light is projected to the liquid sample. The concentration of protein is determined from the intensity of light transmitted through the sample or scattered from the sample. In a urinalysis, an angle of rotation of the sample is measured before the sample is opacified, and in addition, intensity of the transmitted light or scattered light of the opacified urine is measured, whereby the urine sugar and protein levels are obtained.