To provide a sprocket with a simple structure and light weight excellent in corrosion resistance and wear resistance of a key groove part, and a sludge scraping-up device using the sprocket.

A sprocket body 3 is a circular disk-like member having a plurality of teeth 5 on its periphery, and a boss hole 11 penetrating through the sprocket 1 is provided in its center. The boss 7 is a member having a shaft hole 15 penetrating through the boss 7 in its center. A plate installation part 19 is arranged on an end surface of the boss 7. The sprocket body 3 is held by a pair of bosses 7, and a plate 9 installed in the plate installation part 19 is held between the boss 7 and the sprocket body 3. The key groove 27 is arranged on a bottom surface of the plate 9 and is a groove with rectangular cross section having a release part in the shaft hole 15.