To provide a device for purifying bathtub water provided with an ultraviolet ray sterilizing apparatus for flowing water, in which an ultraviolet ray lamp is protected by an outer tube comprising a quartz tube, which is generally manufactured, to reduce cost and completely form a watertight seal.

In an ultraviolet ray sterilizing apparatus provided in a device body, an ultraviolet ray lamp 21 protected by an outer tube 19 is put into a jacket 14, through which water flows from a water inlet port 15 to a water outlet port 16. At that time, in order to attach the outer tube 19 to a jacket cover 17 through an elastic seal tube 30 with flange, a straight base portion 31 of the tube 30 is fitted in the top end of the outer tube, while the portion 31 is tightened using a hose band 33, and a horizontal flange 32 is attached to the jacket cover 17 in such a manner that the flange 32 is pressed against the jacked cover by means of a push plate 34 and screws 37.