A canopy is removably secured to the container body, is respectively fitted with: a base of a water level indicator for detecting a remaining amount of liquid reducing agent; a base of a densitometer for detecting a concentration of a liquid reducing agent; and a heat exchanger which surrounds detection portions respectively suspended from the bases of the water level indicator and the densitometer, and through which is circulated engine coolant, to perform heat exchange with the liquid reducing agent, and a supply port and a return port for the liquid reducing agent are respectively formed in the canopy. The water level indicator, the densitometer, the heat exchanger, and the supply port and the return port for the liquid reducing agent are formed integrally with the canopy, so that the water level indicator and the densitometer can be dismounted together with the canopy at the same time.