Diffusion pairs of RuO2 and glass were prepared as model specimens for composite thick-film resistors and chemical and electrical microanalyses were carried out at the interface to clarify the piezoresistive mechanism of the resistors. The diffusion of ruthenium into glass was examined with SIMS and the chemical structure was analyzed with EELS. In situ measurement of resistance change was conducted as a function of applied load with a microprobe on the RuO2-glass interfaces locally by conductive AFM. The glass with higher piezoresistive sensitivity was found to have larger amount of diffused ruthenium, which has trivalent and tetravalent states in the glass. The originally insulative glass showed electrical conductivity by the diffusion of ruthenium. Furthermore, load-dependent conductivity change was observed near the interface. The analysis suggests that the conduction mechanism is variable range hopping and that the strain-derived electronic state changes must be the origin of the piezoresistive effect of the thick film resistors.