PURPOSE: To provide a compact hot/cold water mixing device that can reduce the power consumption and the size of a driving means for a mixing valve body which has a piston and also can perform the control of the water flow rate and the hot/cold water switching.

CONSTITUTION: This hot/cold water mixing device is provided with a pilot pressure changing means 40 which changes the conducting areas of a cold water primary flowing path 24, a hot water pilot pressure inducing path 34, and a cold water-side pilot pressure inducting path 35 and a flow rate control/hot-cold water switching means 41 which changes the sectional areas of a mixing flow path 43, a hot water secondary flowing path 36, and a cold water secondary flowing path 38. Then the hot water temperature set by a setting means 42 is obtained together with the kind and the flow rate of the discharged fluid.