Provided is a device capable of easily and accurately detecting a vibration period when, for example, an earthquake occurs. When a quartz-crystal plate bends upon application of a force, capacitance between a movable electrode provided at its tip portion and a fixed electrode provided on a vessel to face the movable electrode changes, so that an oscillation frequency of the quartz-crystal plate changes according to this capacitance. Therefore, when the vessel is vibrated, there appear a first state where the quartz-crystal plate ends to approach the fixed electrode and a second state where the quartz-crystal plate is in an original state or bends to be apart from the fixed electrode. Accordingly, an oscillation frequency corresponding to the first state and corresponding to the second state alternately appear, and therefore, it is possible to find the period (frequency) of the vibration.