A sheet heater that includes a sheet article composed of a conductive resin composition containing a conductive material and a resin, and a pair of metal plate electrodes, each of the electrodes being bonded to each of the ends of the sheet article, wherein when elements of the sheet article are detected at a portion 1 μm depth from a surface of the metal plate electrode, a peak area ratio of silicon (Si) to metal ion (M) is 1/100 to 1, the metal ion M being most abundant of all metal ions detected at the portion, the peaks being obtained by measuring an X ray generated at the portion by applying an X ray to the portion with the scanning electron microscope-energy dispersive X-ray spectrometer.