An inline concentration measurement device comprises: a measurement cell main body with a gas flow path formed; a light incident part with a window member connected to the main body; and a light receiving part with a window member connected to the main body, wherein the gas flow path includes a gas flow path for an optical path extending straight between the window members of the light incident part and the light receiving part, a first communication part making a gas inlet formed in the main body communicate with the gas flow path part for the optical path, and a second communication part making a gas outlet formed in the main body communicate with the gas flow path part for the optical path, and the first communication part obliquely extends from the gas inlet towards the window member of the light incident part.