To provide a method for supplying a modifying agent for removing residual chlorine contained in tap water, such as vitamin C, to a user of a water purifier or a shower head.

This method for supplying a tap water modifying agent is adapted for a water purifier or shower head 1. In this method, a cartridge 10 for storing a modifying agent for removing residual chlorine contained in tap water, such as vitamin C15, is provided. A modifying agent is poured into the tap water from the cartridge 10, and, when the modifying agent in the cartridge 10 is used up, the modifying agent is replenished in this cartridge 10. The modifying agent is stored in a hermetically sealed vessel 100 for supply to a user. The user replenishes the modifying agent from the vessel 100 in the cartridge 10.