High-frequency ceramics containing SiO2, Al2O3, MgO, ZnO and B2O3 as constituent components, said ceramics comprising: 30 to 50% by weight of a crystal phase containing ZnO and Al2O3; 5 to 15% by weight of a crystal phase containing SiO2 and MgO; and 40 to 60% by weight of an amorphous phase comprising substantially SiO2 or SiO2 and B2O3; wherein the content of the SiO2 crystal phase is suppressed to be not larger than 6% by weight. The ceramics has a dielectric loss at 60 GHz of not larger than 15×10−4, exhibiting excellent high-frequency characteristics, and is very useful as an insulating substrate for the wiring boards that deal with high-frequency signals.