To provide a lightweight waterproof and dustproof key sheet which is used for a portable electronic apparatus, integrates an elastic body with a reinforcement plate, and prevents the elastic body from coming around to a rear surface of the reinforcement plate, and to provide a manufacturing method of the waterproof and dustproof key sheet.

A reinforcement plate 32, made of a PC resin, is placed on a lower die of an injection mold die, and silicon rubber is injected and fill the die with the reinforcement plate pressed against the lower die by using multiple pins provided on a lower surface of the upper die to obtain a key sheet 12 where the reinforcement plate 32, made of the PC resin, closely contacts and is integrated with the elastic body 22 made of the silicon rubber. In the key sheet 12, a pressure welding part is provided on an upper surface end part or a side surface of the elastic body 22. The welding part is tightly pressure welded to a lower surface of the cover 62 or a side wall inner surface.