PURPOSE: To attain waterproofing without spoiling operability by providing a main body with a lens holding plate which holds plural diopter adjusting lenses which differ in refractive index and an operation number rotatably, and linking the operation member with the lens holding plate through a transmission means and providing a packing member between the operation member and the opposite part of the main body.

CONSTITUTION: A diopter adjustment system 20 consists of the lens holding plate 22 constituted by providing a gear 26 to the outer peripheral surface while holding plural, e.g. four diopter adjusting lenses 21 which differ in refractive index to a plate surface in a peripheral direction and an operation ring 23 (corresponding to the operation member) provided between a case 11 and a cover 12. Then, O rings 34 (corresponding to packing members) made of elastic materials are interposed between respective flanks of the operation ring 23 of the 1st ocular part 6, and internal surfaces of the cover 12 and the top surface of the case 11 (corresponding to the main body part) to attain the waterproofing among the operation ring 23, cover 12, and case 11. Further, the 2nd ocular part 8 is made waterproof on the whole by, for example, the O rings, an adhesive, etc., and the whole of a display device 2 is made waterproof.