To provide a control method and a control device of ER fluid capable of controlling conformation of polymer in ER fluid made of polymer electrolyte aqueous solution and easily controlling viscosity of the ER fluid, with a simple constitution.

A control device of ER fluid includes: an ion supply source 12; an electrode 14 which forms an electrical field to the ion supply source 12; and a filter 16 which is brought into contact with the ion supply source 12 and is formed of a number of fine pores which allow ions to pass therethrough. The device includes ER fluid 20 made of polymer electrolyte aqueous solution which is stored in a container 22 on the side opposite to the ion supply source 12 via the filter 16. An electric field is formed on the ion supply source 12 by applying a voltage to the electrode 14, and the ions in the ion supply source 12 are caused to pass through fine pores of the filter 16 and are supplied to the ER fluid 20. A conformation of polymer in the ER fluid 20 is changed and the viscosity of the ER fluid is controlled.