Described are a urine pretreatment agent, a urine pretreatment method, and a urinary protein quantitation method which reduce or cancel the influences of urine pH variations, cancel the influences of precipitates of urinary inorganic salts, and solubilize membrane proteins. The urine pretreatment agent for urinary protein quantitation, includes a buffer, a chelating agent, and a surfactant; the urine pretreatment method includes a step of mixing 10 to 1000 parts by mass of the urine pretreatment agent of the present invention with 100 parts by mass of urine; and the urinary protein quantitation method includes steps of: mixing 10 to 1000 parts by mass of the urine pretreatment agent with 100 parts by mass of urine; and then measuring the protein concentration.