To provide a cap structure of an urea water tank in which urea is replenished to an urea water tank even though the replenishing nozzle of an urea water can't directly reach a replenishing port of the urea water tank due to the shape of a rack device, and which especially can be used preferably to the urea water tank already disposed in a vehicle.

The cap structure of an urea water tank includes a cap 8 that is engaged with or detached from a replenishing port 31 in an urea water tank 3, a hose 5 which is inserted into the replenishing port 31 and formed with an elastically deformable material, a first flange part 52 at one end of the hose 5 that prevents passing through in the tank inside direction, a second flange part at another end of the hose 5 whose diameter can be reduced and that can be deformed if the urea water passes through the replenishing port 31 and is engaged with the replenishing port 31 after the diameter returns to a normal state in a free condition after the passage of the urea water, and an engaging and releasing mechanism that is provided in the cap 8 and has an operation part where engaging and releasing with/from the cap 8 and the first flange part 52 are performed.