A wafer heater for use in a semiconductor producing apparatus or the like. The heater includes a discoidal substrate made of a dense ceramic, and a resistance heating element buried in the substrate. The surface of the substrate other than that surface upon which a wafer is to be placed for heating is a flat surface. A heating unit is also disclosed, which includes such a heater in a chamber for the semiconductor-producing apparatus. The heating unit further involves a hollow sheath of which inner pressure is not substantially varied even when the pressure inside the chamber changes and is joined to the heater, and a thermocouple inserted into the hollow sheath. Further, a projecting support portion may be provided on the surface of the substrate other than that surface upon which the wafer is to be placed for heating, and lead wires connected to the resistance heating element, wherein when the heater is placed in the chamber, the projecting support portion forms a gas-tight seal between the chamber, and the lead wires are taken out from the chamber such that the lead wires may not substantially be exposed to an inner space inside the chamber.

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