An exercise device, such as a trampoline, that is foldable with long-lasting rigidity in the framework. A measurement signal produced by the signal generator is made analogous to a half-wave waveform, and the level detector detects the level of a waveform analogous to a half-wave of the signal to be detected. The stem member is connected to the body part so as to have a position co-planar with the body part and a position inclined to the plane of the body part.