PURPOSE: To check any possible deterioration in a heater element as well as to uniformize a heating distribution by stacking each surface positive characteristic heater on both sides of an insulator film, and connecting each set of electrodes to the top as in a series circuit.

CONSTITUTION: Each of surface positive temperature characteristic heaters 2 and 3 consisting of a conductive material powder layer formed with a high polymer material binder is laminated on both vertical sides of an insulator film 1. Each set of electrodes 4, 5 formed on these heaters 2, 3 is connected to a series circuit, through which a surface heater element is constituted. At this time, electrode positions at both vertical sides am dislocated to each other so as to make each position of both electrodes 6, 7 come to position corresponding to an interval between electrode positions installed on the top side. If so, a temperature difference between these electrode positions becomes lessened, so a uniform heating distribution is shown, thus any possible deterioration in the element when the surface heater element is energized with current for hours is checked.