This study attempted to clarify how the presence of micro-bubbles, their particle diameter and density, and the circulation of water affect the thermoregulatory function, thermal comfort and skin cleansing of humans during hot water bathing. Nine healthy males (20.8 ± 0.5 years old) bathed for 10 min. During the experiment, we measured their tympanic temperature and leg skin temperature, forearm skin blood flow, chest sweat rate, leg heat flow as a thermoregulatory function index, saliva amylase, and subjective whole thermal sensation and comfort as a thermal comfort index, blood pressure and heart rate as a physiological index, and skin luminosity after putting evaluation smear on skin surface as a washing index. The bathing water was heated at of 40 °C and we examined four conditions: no micro-bubbles, a great quantity of bubbles released intermittently, a small quantity released continuously, and a small quantity released intermittently. We found that a small quantity of micro-bubbles in the bathing water had a greater thermal effect than water without micro-bubbles, because of the circulation of water. In contrast, we found that the use of micro-bubbles in great quantity had only a moderate thermal effect, but it gave rise to some inhibition of sympathetic activity. We think that this effect is caused by the circulation of water and to some air space formed on the skin surface. We also found that micro-bubbles enhance skin cleansing, especially in the condition of great quantity of bubbles and high circulation of water.