This invention provides a method for measuring human megalin that can be performed in a simpler manner within a shorter period of time than is possible with conventional techniques, and that can also quantify human megalin. This invention also provides a method that enables diagnosis of functional diseases, which are specific to cells, tissues, or organs, in a site-directed manner at an early stage. Measurement of human megalin enables detection of a disease in an organ in which megalin expression is observed.