To clean underground water under a reducing atmosphere containing ammonium nitrogen, ferric iron, and manganese ion while suppressing a supplying quantity of hypochlorous ion.

An apparatus 1 for cleaning the underground water includes: a water taking channel 10 for pumping up the underground water; a cation exchanging device 20 directly connected to the water taking channel 10 and for obtaining treated water by treating the underground water by cation exchanging bodies; a water sending channel 30 for sending the treated water; a chemical agent feeding device 50 arranged in the water sending channel 30 and for feeding hypochlorous acid aqueous solution to the treated water; a measuring device 60 arranged downstream of the chemical agent feeding device 50 in the water sending channel 30 and for measuring the hypochlorous ion concentration of the treated water; a regenerating device 40 for regenerating the cation exchanging bodies of the cation exchanging device 20; and a control device 70 for operating the regenerating device 40 when the measuring result in the measuring device 60 lowers below a reference value set in advance.