To control the quantity of water needed at the time of back washing of a cleaning tank for cleaning bathtub water to be the optimum quantity corresponding to the dirty state of the cleaning tank.

This bath water cleaning apparatus comprises a pump 1 for circulating bathtub water, a purifying tank for filtering fine dust and impurities, which are dirt, from bathtub water, and a switching valve 9 for switching the flow routes for passing bathtub water in a prescribed direction. Bathtub water is cleaned by circulating bathtub water in the purifying tank in a right direction and the flow routes are switched by the switching valve and bathtub water is passed through the cleaning tank in the reverse direction to discharge impurities adhering to the purifying tank and to back-wash the cleaning tank. In this case, the apparatus is provided with a dirt detecting means 32 for detecting how much the bathtub water is polluted and also a back-washing completion determining means 45 for automatically finishing back washing of the cleaning tank when the detected polluted state of the wastewater discharged at the time of back-washing of the purifying tank meets prescribed conditions.