1. A heater, comprising:a first ceramic substrate having a heating face on a first main face thereof;
a second ceramic substrate disposed so that a first main face of the second ceramic substrate covers a second main face of the first ceramic substrate;
a heat generating resistor disposed on the first main face of the second ceramic substrate; and
an adhesive layer which bonds the first ceramic substrate and the second ceramic substrate while covering the heat generating resistor
wherein the first main face of the second ceramic substrate includes a groove section, and the heat generating resistor is disposed on a bottom face of the groove section along a profile of the groove section,
wherein a thickness of the heat generating resistor is smaller than a depth of the groove section,
wherein the thickness of the heat generating resistor becomes gradually smaller toward a center side in a width direction of the groove section, and
wherein the second ceramic substrate is 1 to 10 mm in thickness.