To provide a stuck stain measuring instrument capable of rapidly measuring the amount of the stain stuck to a piping to obtain an accurate measured value and properly performing the automatic control of water treatment, and a stuck stain measuring method using it.

A light emitting element 2 and a light detecting element 3 are attached to the outer surface of a measuring cell 1 made from a transparent acrylic resin pipe in mutually opposed relationship. A supply pipe 4 is connected to the lower end of the measuring cell 1 and also connected to a white water storage tank through a flow channel changeover four-way valve 5 and a supply pump 6. The flow channel changeover four-way valve 5 is controlled by a control device 9 and made possible not only to supply water into the measuring cell 1 but also to drain the water in the measuring cell 1.