The object of the present invention is to prevent the heat of the hot water contained in the bath to prevent the hot water of a suitable temperature for a long time.;In order to achieve the above-mentioned object, the present invention has a feature that a hollow body 12 formed between a pair of plate bodies 11 and 11 ' 13 '13' between the plate bodies 11 and 11 'to support the central portion of the bathtub 1 against the double tub 1 by a load acting as a lower portion.;Such a design can increase the metabolism of the body, increase the effect of health, and can achieve energy saving effect in particular.
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