To provide a urea water tank easily forming or attaching components such as a water filling opening and a pipe/sensor unit, facilitating machining of a tank body, and easily adjusting capacity of the tank in accordance with the required amount of urea water stored.

The tank body is so formed that a tank barrel member, a stainless steel component mounting plate and two stainless steel sideboards are welded or joined to each other by brazing. The tank barrel member includes a front surface portion, a bottom surface portion, a rear surface portion and an upper surface portion which are formed by longitudinally bending a rectangular stainless steel plate and has a gap between the front surface portion and the upper surface portion. The stainless steel component mounting plate is bridged over the gap of the tank barrel member, is formed with respective holes for mounting the pipe/sensor unit and a gauge for confirmation of quantity of urea water, and has an inclined plate portion having the water filling opening integrally formed perpendicularly to an inclined surface. Each of the two stainless steel sideboards is formed into a planar shape covering both the side portions of a square cylinder composed of the tank barrel member and component mounting plate, and includes a joining bent edge portion formed along a periphery.