PURPOSE: To obtain the title method whereby its formation can be made small and simple.

CONSTITUTION: With manual operation or through an automatic control device 50, a liquid feed pump 42 is driven, following which a hydrazine stock solution feed pump 48 is driven, and thus a definite amount of hydrazine stock solution is fed to water in a feed-water line 44. This hydrazine solution is fed to a process plant 34 through a solution pipe-line 44'. After the solution is fed, the liquid pump 42 is stopped.