The invention provides a percarboxylic acid concentration-determining device capable of determining the concentration of percarboxylic acid contained in an aqueous solution for chemical sterilization or disinfection that contains a percarboxylic acid as an active ingredient; and an indicator solution for use in preparing the device. The indicator solution contains a starch, an iodide, a thiosulfate, a water-soluble organic solvent, and water, with the starch, the iodide, the thiosulfate, and the water-soluble organic solvent being present in the following proportions, based on the total amount taken as 100 mass %: (a) starch: 0.01 to 5 mass %; (b) iodide: 0.01 to 5 mass %; (c) thiosulfate: 0.01 to 10 mass %; and (d) water-soluble organic solvent: 0.1 to 40 mass %, wherein the total amount of (a), (b), and (c) is greater than 0.6 mass %.