To enhance accuracy of a mixing ratio and to widen a measuring range by increasing accuracy of a flow rate at a less flow rate side.

This method mixes at least two fluids of a first chemical MA and a second chemical MB. In this case, during the continuous transfer of the first chemical MA by a pressure, a set quantity of the second chemical MB is forcibly fed with an interval in accordance with a flow rate of the first chemical MA to feed into the first chemical MA. This device is provided with a necessary pump 11 for forcibly feeding the first chemical MA, a rotary encoder 17 for measuring the discharge flow rate, and a metering pump 25 constant in the flow rate by one time of a discharge work. A control device 50 allows the metering pump 25 to discharge with an interval in accordance with a discharge flow rate of the measuring pump 11.