An exercise device which is in a chair shape is provided with a seat. The seat is connected to a base portion in a bendable way (for example, a spiral spring is utilized), so that the seat can bend towards multiple directions. The base portion of the chair can be a plurality of support feet, so that the chair can be fixed on the ground. The chair further comprises grip bars provided with grips, wherein the height, relative to the ground, of the grips is no larger than the height, relative to the ground, of the seat. When a user bends the body, the seat is bent relative to the base portion, and the exercise device brings convenience for the user to exercise core muscles of the user.