A foldable knife includes a handle, a frame pivotally connected to the handle, a cartridge pivotally connected to the frame, a leaf spring attached to the frame, a blade-driving device, a blade-positioning element and a button. After having an annular supporting portion that bulges inward of the battery case support an opening-sealing member, the opening-sealing member is fixed through caulking by bending the opening end portion of the battery case inward, whereby a battery is manufactured. Each FBAR has opposed planar electrodes and a layer of piezoelectric material between the electrodes. This changes length of a path area of an induced current, flowing in an induced current film.