Water-miscible functional fluid, undiluted stock of water-miscible functional fluid, sterilizer for water-miscible functional fluid, and method of sterilizing water-miscible function fluid

A water-miscible functional fluid comprising:

• a neutral photosensitizer having an isoalloxazine skeleton;
• an antioxidant;
• a rust inhibitor;
• and
• a surfactant, wherein
• the neutral photosensitizer is riboflavin sodium phosphate and/or riboflavin butyrate;
• the antioxidant is at least one selected from the group consisting of D-α-tocopherol, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), 1,3,5-Tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, tris(2,4-di-tert-butylphenyl)phosphite, tributyl phosphite, a secondary amine, and a tertiary amine,
• a content of the neutral photosensitizer is 20 ppm to 1000 ppm, and
• a content of the antioxidant is 0.0001 parts by weight to 0.03 parts by weight.