To obtain a measuring apparatus by which the concentration of a fluid as a sample can be measured with good accuracy according to the property of the fluid.

A first measuring body 21a, a second measuring body 21b and a third measuring body 21c are connected to a treatment-liquid supply pipe 6 via couplings 20 and intermediation pipes 6a at prescribed intervals in a direction in which a treatment liquid flows. Light transmission parts 22 in which the length of the optical path of light to be transmitted is made different are installed at them. According to a concentration which is set by a concentration setting device, a first optical-path changeover mechanism 15 is changed over, only one out of the light transmission parts 22 at the first, second and third measuring bodies 21a, 21b, 21c is irradiated with light from a light source, and the light which is transmitted through the light transmission part 22 is incident on a photodetector 18.