A heater includes a ceramic body of bar shape; and a heat-generating resistor disposed in an inside of the ceramic body, when viewed in a transverse section, the heat-generating resistor including at least one step portion provided in an outer periphery part of the heat-generating resistor, the at least one step portion having such a shape that semicircular portions bisected in a diametrical direction of the heat-generating resistor deviate from each other along the diametrical direction.