Device for measuring a concentration of a reducing agent, in particular urea, in a solution, in particular an aqueous solution, comprising: a tank housing for storing the solution in an interior space of the tank housing; a sensor device for emitting a first measurement signal along an emission direction into the interior space and for receiving reflections of the first measurement signal, the sensor device being mounted on an outer side of the tank housing facing away from the interior space a reflector component in the interior space having a first reflector which is arranged at a distance from the sensor device in the emission direction and reflects a first part of the first measurement signal back to the sensor device opposite to the emission direction, and a second reflector which is spaced apart from the sensor device in the emission direction by a distance further than the first reflector and which reflects a second part of the first measurement signal back to the sensor device opposite to the emission direction, wherein the tank housing has an indentation extending substantially in the direction of the interior, through which indentation an installation recess for receiving the sensor device is formed on the outside of the tank housing, wherein the installation recess is bounded by a bottom wall of the recess and by a side wall of the recess, and wherein the sensor device is arranged in the installation recess, characterized in that an opening of the installation recess forms an outwardly open end of the installation recess facing away from the bottom wall of the recess and the interior of the tank housing, and in that the indentation has a tub-like basic shape, the sensor device emitting the first measurement signal along the emission direction at least essentially perpendicular to the side wall of the indentation. a tank housing for storing the solution in an interior space of the tank housing; a sensor device for emitting a first measurement signal along an emission direction into the interior space and for receiving reflections of the first measurement signal, the sensor device being mounted on an outer side of the tank housing facing away from the interior space a reflector component in the interior space having a first reflector which is arranged at a distance from the sensor device in the emission direction and reflects a first part of the first measurement signal back to the sensor device opposite to the emission direction, and a second reflector which is spaced apart from the sensor device in the emission direction by a distance further than the first reflector and which reflects a second part of the first measurement signal back to the sensor device opposite to the emission direction, a first reflector which is arranged at a distance from the sensor device in the emission direction and reflects a first part of the first measurement signal back to the sensor device opposite to the emission direction, and a second reflector which is spaced apart from the sensor device in the emission direction by a distance further than the first reflector and which reflects a second part of the first measurement signal back to the sensor device opposite to the emission direction, wherein the tank housing has an indentation extending substantially in the direction of the interior, through which indentation an installation recess for receiving the sensor device is formed on the outside of the tank housing, wherein the installation recess is bounded by a bottom wall of the recess and by a side wall of the recess, and wherein the sensor device is arranged in the installation recess, characterized in that an opening of the installation recess forms an outwardly open end of the installation recess facing away from the bottom wall of the recess and the interior of the tank housing, and in that the indentation has a tub-like basic shape, the sensor device emitting the first measurement signal along the emission direction at least essentially perpendicular to the side wall of the indentation.