A concentration measuring device for determining a concentration of gas or particles in a measurement volume includes at least one housing having an opening for communication with the measurement volume, a light source for transmitting measurement light through the housing into the measurement volume, a light receiver for receiving the measurement light after its passage through the measurement volume and an evaluation unit which is designed for determining the concentration of gas or particles from the measurement light received at the light receiver. In accordance with the invention at least one body of solid material is arranged in the at least one housing such that the measurement light path largely passes through the at least one solid body within the housing, with the portion of the measurement light path within the at least one housing not passing through the at least one solid body having a specified total length.