An airbag for a steering wheel is configured to cover an upper surface of the steering wheel generally all over when inflated. The airbag includes a driver-protection portion that is configured to be deployed towards a driver's seat and restrain a driver, a central protruding portion that is disposed in a vicinity of a center of the driver-protection portion and protrudes towards the driver's seat, a circumferential protruding portion that is disposed in a vicinity of an outer circumferential edge of the driver-protection portion and protrudes towards the driver's seat, and a small protruding portion that is disposed between the central protruding portion and circumferential protruding portion and protrudes towards the driver's seat. The small protruding portion is smaller in protruding amount than the central protruding portion and the circumferential protruding portion.