A circuit substrate comprises a glass substrate 16, through-holes 18 formed through the glass substrate 16 and via electrodes 20 buried in the through-holes 18. An opening width of the through-holes 18 is minimum inside the glass substrate and is increased toward both surfaces of the glass substrate 16. Accordingly, the detachment of the via electrodes 20 can be prevented without increasing the surface roughness of the inside walls of the through holes, and stresses generated in the core substrate can be mitigated.