A multi-layer ceramic substrate is formed of a plurality of glass-ceramic layers. The glass-ceramic layers (partly not shown) contain amorphous glass and alumina (Al2O3), and interconnection patterns of silver are formed in the surfaces of the glass-ceramic layers. The amorphous glass is anorthite (CaAl2Si2O8), for example. With the multi-layer ceramic substrate, the upper limit of the firing temperature is set so that the degree of crystallinity is 12% or less. The lower limit of the firing temperature is set so that the multi-layer ceramic substrate has a sintered density of 95% or more with respect to the sintered density that the multi-layer ceramic substrate exhibits when the degree of crystallinity is 25%.