Electrorheological fluid composition containing inorganic/organic composite particles

An electrorheological fluid which possesses high electrorheological effects, has superior storage stability and longevity, does not damage instrumentation, is unaffected by ambient temperature or humidity, has stable current values, and low power consumption, is disclosed. In the electrorheological fluid, inorganic/organic composite particles comprising a core comprising inorganic polymeric compound, and a shell comprising an inorganic material particle possessing electrorheological properties, are dispersed in an electrically insulating medium. The surfaces of these inorganic/organic composite particles may be polished, and furthermore, the inorganic/organic composite particles are preferably produced by means of a method in which the core and the shell are formed simultaneously.