A core assembly for a pneumatic tire, which is fitted annularly to an outer peripheral surface of a well portion of a rim, and which rotates should the pneumatic tire collapse owing to a decline in its internal pressure, as an outer surface of the core assembly comes into contact with an inner peripheral surface of a crown portion of the pneumatic tire and an inner peripheral surface of the core assembly slides over the outer peripheral surface of the well portion of the rim. The core assembly includes a sliding portion formed of a resin material having good sliding properties and high heat resistance, and a main body to which the sliding member is secured and which is formed of a material having a high rigidity and a low specific gravity. Accordingly, since the surface of the core assembly contacting the rim is constituted by the sliding member, the core assembly is rotated smoothly when the internal pressure of the pneumatic tire declines, and the overal weight of the core assembly is reduced.