PURPOSE: To provide the convenient method and device capable of analyzing an additive in an electrolyte tending to change on aging.

CONSTITUTION: A loop injector 2, a mixing coiled-tube 4, a primary reaction 6 coiled-tube 6 held in a thermostatic bath 5, a secondary reaction coiled-tube 7 and an abosorptiomemter 8 provided with a flow cell are used, and the additive in the electrolyte is allowed to react with iodine to obtain the additive concn. Consequently, a skill is not needed in the determination of the additive in the electrolyte, the analysis is conducted even by an inexperienced person, the analyzing time (the actual working hours of an operator) is reduced to 1.5 to 2hr from 3.5 to 4hr spent in the conventional method, and rapid analysis can be performed even on the site.