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A new process for a highly reliable ceramic substrate with electric circuit has been developed for power devices of automotive applications. The circuit conductor has high conductivity to accept a current of 10 to 200 Amperes, and has strong joining strength to ceramic substrates. The conductor is made of sintered copper powders with several kinds of diameters to get a porous structure, for reducing Young's modulus and thermal stress between the circuit conductor and ceramic substrate. Sintering shrinkage of the circuit conductor has been restrained, and joining strength to the ceramic substrates kept strong by an anchor effect. The copper circuit conductor realized electric resistivity of 2.3×10-6 ohm-cm, and coefficient of thermal conductivity, 290 W/mK. We have also confirmed superior reliabilities in the joining strength and conductivity by thermal cycling tests.