PURPOSE: To provide a method and device for avoiding wasteful switching at normal time and for non-break-controlling a liquid chemicals injecting pump which is redundantly provided in preparation for the injection interruption of liquid chemicals such as air lock without a delay for the resumption of liquid chemicals injection after a trouble occurs.

CONSTITUTION: A process controller 100 receiving a liquid chemicals injected process quantity transmission signal WFR, a liquid chemicals density set signal SP1 and a liquid chemicals injecting quantity measuring signal PV1 and outputting a liquid chemicals injecting quantity operation signal MV2 and a redundancy operation device 200 receiving the liquid chemicals injecting quantity operation signal MV2 and outputting plural redundancy operation signals MV2A and the like are provided. The redundancy operation device 200 calculates the plural redundancy operation signals MV2A as proportional response quantity which is sequentially distributed to the liquid chemicals injecting quantity operation signal MV2, supplies the total response quantity of the plural redundancy operation signals MV2A as proportional response quantity which is sequentially distributed by including the lacking time of one of the plural redundancy operation signals MV2A and operates the plural liquid chemicals injecting pumps by using the respective redundancy operation signals MV2A.