A contact inspection method and device in which a detection element which produces a detection output according to vibration of a rotating body is mounted on the rotating body to detect contact of an object with the rotating body, more particularly a method and device suitable for evaluating mechanical characteristics of a rotating magnetic disk and a slider in a magnetic disk device are provided. When a failure has been detected in the neutral line, the controller modifies the voltages for the lamps in the signaling system for better illumination of the lamps during the failure condition, such as to the highest voltage available from a battery in the system. The two optical members 12, 14 are arranged such that the optical fibers respectively constituting each of the optical members 12, 14 have a gap therebetween widening from the entrance end face 12a, 14a side to the exit end face 12b, 14b side. And, the extension region connects one of the source region or the drain region.