Abstract: An abdomen exercising apparatus is disclosed having a back rest supported on two curved supports at two opposite sides of a base thereof by two cranks and two elastic elements, which elastic elements are respectively connected between the curved supports and the cranks to constantly pull up the cranks for permitting the back rest to be disposed at a top position perpendicular to a cushion on the base. The back rest is moved backwards to a bottom position closely attached in line with the cushion as the player sits on cushion and then lies down, and automatically pulled up to the top position by the elastic elements through the cranks as the player sits up. Metal fixing rods may be connected between the curved supports and the cranks to fix the back rest in position.