Abstract: A concentration measurement apparatus capable of accurately measuring the concentration of suspended matter in a liquid is provided.A transmitter 1 which transmits sound waves, light waves, or the like, and a receiver 2, which is disposed in opposition to this transmitter 1, are provided, small pipes 3 and 4, which enclose the transmitting surface position of the transmitter and the receiving surface position of the receiver, and which are opened in the direction of transmission and the direction of reception, respectively, are provided, and a mechanism for filling these small pipes with a clean liquid or gas is provided.The transmitting surface of the transmitter and the receiving surface of the receiver are not contaminated by means of the deposition or generation of suspended matter, so that it is possible to conduct the accurate measurement of the concentration in a constant manner.