Although heat-assisted plasma treatment enables drastic improvement of the adhesion property of polytetrafluoroethylene (PTFE), plasma-treated PTFE does not strongly adhere to any adherend. To clarify which rubber compounding agents positively affect the adhesion strength of a plasma-treated PTFE/rubber assembly, six types of unvulcanised rubbers were prepared and thermally compressed to a plasma-treated PTFE sheet. Thus, it was found that SiO2 addition to rubber drastically increased the adhesion strength of a plasma-treated PTFE/rubber assembly and cohesion failure of rubber occurred with large fractions of SiO2 although no adhesives were used. To confirm the reaction between plasma-treated PTFE and SiO2 powder, X-ray photoelectron spectroscopy (XPS) measurements were performed for the thermally compressed SiO2/PTFE assembly after repeated washing. The XPS results indicated that hydrophilic SiO2 powder strongly adhered to the plasma-treated PTFE, whereas hydrophobic SiO2 powder did not adhere to the PTFE. In this paper, a model was proposed for a possible mechanism of strong adhesion of a PTFE/rubber assembly through both hydrogen and covalent bonds between silanol groups of the SiO2 powder surface in the rubber and hydroxyl or carboxyl groups on the plasma-treated PTFE.