A circuit substrate has three wiring layers, wherein a signal line is formed in a first wiring layer; a ground plane is formed in a second wiring layer; a resonant line is formed in a third wiring layer. A circumferential slit is formed in the ground plane, wherein an island electrode separated from the ground plane is formed inside the slit. The left end of the resonant line is connected to the island electrode through an interlayer-connecting via, while the right end of the resonant line is connected to the ground plane through an interlayer-connecting via. A transmission line (or a microstrip line) is formed using the signal line and the ground plane, and therefore a complex resonator is formed to embrace the transmission line. This achieves band elimination with regard to a signal component of a resonance frequency among signals propagating through the microstrip line. Thus, it is possible to form a noise suppression structure without mounting additional parts on the circuit substrate, and therefore it is possible to effectively eliminate power distribution noise and noise propagating through the signal line with a small and simple configuration.