PURPOSE: To improve reliability and prolong life by putting a light emission element and a photo-electric element in the same housing, directly attaching them to a lower substrate etc., reflecting light emitted from the light emission element by a reflecting plate and receiving it by the photo-electric element.

CONSTITUTION: The light emission element 152a and photo-electric element 152b are put in the same housing 152. Angle of the elements 152a and 152b is so determined that light emitted from the light emission element 152a is reflected by fitting plates 6 attached at an interval and received by the photo-electric element 152b when the notched part 5a of a floppy disk 5 in the housing 152 is not closed. The housing 152 is directly attached to the substrate 3c of a driving motor or lower substrate 77. Thus, wiring and screwing of the write protecting mechanism are eliminated and number of parts is reduced remarkably to improve reliability and prolong life.