To keep bathtub hot water at predetermined temperature always and purify bathtub hot water.

A warmer for bathtub is provided with a case 1 having a flow-in opening 16 and a flow-out opening 17, a heater 5 arranged in the case 1, float switches 4, 35 turning on and off current carrying into the heater 5, a control part 34 controlling current carrying into the heater 5 to keep bathtub hot water at the predetermined temperature, and a purifying part 2 attached detachably to an outer side of the flow-in opening 16. Preferably, the purifying part 2 is constituted by filling a vessel 21 forming a plurality of holes 23 for circulating bathtub hot water with granular material 22 having purifying action and larger than the hole 23 to improve purifying action of the purifying part 2. Here, it is preferable to use at least one substance selected from a group of granite porphyry, tourmaline, charcoal, coral, zeolite, titanium oxide, zirconium oxide, and ion exchange resin and ceramic ball carrying at least one of them as the granular material 22.