A heater which realizes both reduction of power consumption and improvement of durability against thermal shock. The heater includes a ceramic substrate formed of a ceramic material containing alumina as a main component; and a heat-generating resistor provided on the ceramic substrate and having a heat-generating portion and a lead portion, the heat-generating resistor containing, as a main component, one or more metals selected from the group consisting of platinum (Pt), palladium (Pd), and rhodium (Rh), or an alloy of any of these, and a ceramic material which is the same as the ceramic material of the ceramic substrate. In the heater, the ratio of the resistance of the heat-generating portion to the sum of the resistances of the heat-generating portion and the lead portion is 76 to 95%, and the heat-generating portion has a thickness of 1 to 6 μm.