PURPOSE: To easily recognize a limit of a purifying capacity of a purifying device by using an adsorbent or matrix impregnated with a dye.

CONSTITUTION: When water passing through the purifying device exceeds the limit of variation of a fixed ion strength or pH, the dye 17 is released from the adsorbent or matrix. In this way, a warning color is imparted to the water. At this time, the dye 17 is a dye lake adsorbed on a neutral matrix or an acidic dye on a weak basic ion exchange resin, or exists as the dye adsorbed on an acidic ion exchange resin and containing an excess amino or ammonium group.