PURPOSE: To enable easy macroscopic detection of the presence of oxygen based on the change of a color tone by using an oxygen detecting agent prepared by mixing methylene blue, glucose, sodium compound and water.

CONSTITUTION: Methylene blue as a constituent (a) accounting for about 0.001W 1wt% in a detecting agent, glucose as a constituent (b) bearing 5 parts or more to 1 part of the constituent (a), one kind or more of sodium silicate, sodium carbonate, sodium acid carbonate and the like as a constituent (c) bearing 5 parts or more to 1 part of the constituent (b), and water as a constituent (d) accounting for 0.1W 20% in the detecting agent are formed into a composition and mixed into a solid state. The oxygen detecting agent thus produced turns white in the absence of the oxygen, while turning violet through recoloring when it is left in the air. Since this agent can be used repeatedly, the presence of the oxygen in vacuum packing or a closed container employed for preservation of foods or medicines, etc. can be checked up with ease.