A heater includes a sintered assembly and a functional element disposed on one of opposing surfaces of the sintered assembly. The heater is manufactured by a method that includes hot pressing a ceramic powder and a plurality of first slugs and forming the sintered assembly including a ceramic substrate and the plurality of first slugs embedded therein, forming the functional element on the one opposing surface of the sintered assembly such that the functional element is connected to the plurality of first slugs, and forming a monolithic substrate in which the functional element and the plurality of first slugs are embedded.