In a sparkplug voltage probe device, an insulator base is attached to the internal combustion engine, an upper surface of the insulator base having a plurality of grooves in which the corresponding cables are placed. An electrode plate is embedded in the insulator base along the grooves to form a static capacity between the electrode plate and an array of all the cables in the grooves, both of the longitudinal sides of the electrode plate being curled around the outermost cables respectively to compensate shortage of static capacitance so as to form a capacitance adjusting function in order to substantially even a distribution of the static capacity between the electrode plate and the array of all the cables in a direction perpendicular to a length of the electrode plate. A lead wire is provided to electrically connect the electrode plate to a microcomputer.