PURPOSE: To reduce the power consumption and the size of a driving means for a mixing valve body which has a piston.

CONSTITUTION: This hot/cold water mixing device is provided with a hot water valve unit 25 which has a hot water valve body 27, a cold water valve unit 26 which has a cold water valve body 32, a pilot pressure changing means 44 which changes the conducting areas of a hot water pilot pressure inducing path 38 and a cold water pilot pressure inducing path 39 that cover a cold water primary flowing path 24 through a hot water pressure piston 31 and a cold water pressure piston 35. The temperature of the hot water mixed with the cold water that is detected by a temperature detector 46, is compared with the temperature set by a setting means 48, and a proper temperature is obtained by the feedback control.