A composite electronic device includes first and second magnetic substrates, and a functional layer sandwiched between these magnetic substrates, and the functional layer is configured by a common-mode filter layer and a ESD protection element layer. An electrostatic capacitance of the ESD protection elements is equal to or lower than 0.35 pF. The common-mode filter layer includes a first spiral conductor formed on an insulation layer, and a second spiral conductor formed on an insulation layer. DC resistance of a common mode filter is equal to or higher than 0.5Ω and equal to or lower than 5Ω, and an electrostatic capacitance of the ESD protection elements is equal to or lower than 0.35 pF. A width W and a length L of the first and second spiral conductors satisfy a relational equation expressed by √(L/W)