A plasma-polymerized film is formed by depositing and polymerizing a metal element-containing material forming an electrode and a gaseous polymerizable carbon-containing reactant in a plasma atmosphere. The polymerized film contains the metal element Mt and carbon C in an atomic ratio Mt/(C+Mt) of at least 1/100. The polymerized film is prepared under a condition W/(F.M) of at least 10.sup.8 joule/kg wherein W is a power applied for plasma generation in joule/sec., F is a flow rate of the gaseous reactant, and M is the molecular weight of the gaseous reactant, F.M being expressed in kg/sec. Such a polymerized film is advantageously applied as a coating to a surface of a member which constructs or is disposed in a casing in which a tape or disk-shaped medium is received for travel or rotating motion.