A substrate for mounting an electronic component, includes a baseplate having a main surface formed with or without a recess. A ceramic substrate is provided on the main surface of the baseplate and has a smaller size than the baseplate. A metal layer is provided to cover both of the baseplate and the ceramic substrate. The metal layer has a surface remote from the baseplate and the ceramic substrate is made flat, the baseplate is made of a metal-ceramic composite of a metallic material and a ceramic material, and the metallic material in the composite and the metal layer have different compositions. Thus, an integral ceramic circuit board, which is resistant to repeated thermal stress and is superior in reliability can be realized.