A water purifier comprising a pre-treatment filter connectable to an inlet flow path; an auxiliary filter including a filter cap and a filter body, connectable to a first connection flow path; a membrane filter including a filter cap and a filter body, so that water discharged from the auxiliary filter is introduced through a second connection flow path; a post-treatment filter including a filter cap and a filter body, so that water discharged from the membrane filter is introduced through a third connection flow path; a branch flow path that branches off from an upstream side of the first connection flow path and merges on a downstream side of the first connection flow path; an electrolysis module arrangeable along the branch flow path; and an opening/closing valve arrangeable along the first connection flow path so that water flows to the first connection flow path or the branch flow path.