An electronic component module , comprising: a circuit board including a ceramic substrate having a first principal surface and a second principal surface opposite to the first principal surface, a first metal plate bonded to the first principal surface of the ceramic substrate , and a second metal plate bonded to the second principal surface of the ceramic substrate , wherein the ceramic substrate is composed of a silicon nitride sintered compact; at least one electronic component bonded to the first metal plate via a first brazing material layer and operable at a temperature of at least 125°C and 500°C or less, wherein the first brazing material layer is formed of an Ag-Cu based brazing material or an Al-based brazing material having a melting point of at least 600°C; and a base plate bonded to the second metal plate via a second brazing material layer , wherein the second brazing material layer is formed of an Ag-Cu based brazing material or an Al-based brazing material having a melting point of at least 600°C; wherein each of the first metal plate and the second metal plate contains at least one of copper and aluminum as the main constituent, wherein the Ag-Cu based brazing material in the first brazing material layer and in the second brazing material layer contains Ag and Cu at 85% or more by mass in total and further contains at least one selected from Ti, Zr and Hf in a range of not less than 1% nor more than 5% by mass, and the Al based brazing material in the first brazing material layer and in the second brazing material layer contains Al at 90% or more by mass, and wherein the ratio of the thickness of the first metal plate to the thickness of the second metal plate is not less than 50% nor more than 200%. a circuit board including a ceramic substrate having a first principal surface and a second principal surface opposite to the first principal surface, a first metal plate bonded to the first principal surface of the ceramic substrate , and a second metal plate bonded to the second principal surface of the ceramic substrate , wherein the ceramic substrate is composed of a silicon nitride sintered compact; at least one electronic component bonded to the first metal plate via a first brazing material layer and operable at a temperature of at least 125°C and 500°C or less, wherein the first brazing material layer is formed of an Ag-Cu based brazing material or an Al-based brazing material having a melting point of at least 600°C; and a base plate bonded to the second metal plate via a second brazing material layer , wherein the second brazing material layer is formed of an Ag-Cu based brazing material or an Al-based brazing material having a melting point of at least 600°C; wherein each of the first metal plate and the second metal plate contains at least one of copper and aluminum as the main constituent, wherein the Ag-Cu based brazing material in the first brazing material layer and in the second brazing material layer contains Ag and Cu at 85% or more by mass in total and further contains at least one selected from Ti, Zr and Hf in a range of not less than 1% nor more than 5% by mass, and the Al based brazing material in the first brazing material layer and in the second brazing material layer contains Al at 90% or more by mass, and wherein the ratio of the thickness of the first metal plate to the thickness of the second metal plate is not less than 50% nor more than 200%. <EOS>