PURPOSE: To provide the hot water/cold water mixer which can reduce the power of the driving means of a mixing valve body provided with a piston, can miniaturize the driving means and hardly fixes the valve body with the clogging of dust.

CONSTITUTION: This device is provided with a hot water side valve body 29, cold water side valve body 29, hot water side pressure piston 32 linked with the hot water side valve body 28, cold water side piston 33 linked with the cold water side valve body 29 and provided with a pressure receiving area larger than equal in comparison with that of the hot water side pressure piston 32, hot water side pilot pressure introducing path 36, cold water side pilot pressure introducing path 37, and pilot pressure varying means 42 for increasing/ decreasing the communicated area of a water flow-in duct 25. Then, the temperature of mixed hot water detected by a temperature detector 44 is compared with the set temperature of a setting means 46, and the suitable temperature is provided by feedback control.