A novel indicator for plasma sterilization utilizes an azo dye which has been known and used for preparing indicator for ethylene oxide gas sterilization and indicator for heat sterilization, and which exhibits a quick change of color upon the cold plasma sterilization process using an oxidizing gas such as hydrogen peroxide, the change of color being indicated as a clear change of the color tone. The azo dye is used in combination with a compound containing a mercapto group or a dithiocarbamyl group therein as the compound capable of reacting with the azo dye upon the plasma sterilization treatment, and thereby bringing discoloration into the azo dye. Moreover, in combination with them, a polyhydric phenol compound or an aromatic carboxylic acid may be further used as antifading agent for discolored azo dye which is produced during the plasma sterilization process in order to stabilize the occurrence of discoloration.