To provide an opening/closing device which improves anti-corrosion effects of an opening/closing shaft, and an anti-corrosion method thereof.

An opening/closing device 1 comprises an opening/closing machine 2 which is installed in a water passage S or the like and installed above a water level, and an opening/closing shaft 3 of which the upper end side connected to the opening/closing machine 2 is exposed above the water level and of which the lower end side connected to a control valve 23 is positioned below the water level. In the opening/closing apparatus 1, a protecting cover 6 is provided which covers at least around the opening/closing shaft 3 exposed above the water level and of which the inside is shielded from an atmosphere of the water passage S. The protecting cover 6 is fixed to a slab 24 which is supported by the opening/closing shaft 3 via a bearing and of which the upper end constitutes the ceiling of the water passage S. External air is supplied by a compressor 26 or water in the water passage S is supplied by a submerged pump 27 into the inside of the protecting cover 6.