A decoupling device that can be used for millimeter-waves is required to reduce the effect of the parasitic impedance at the grounded terminal of the power supply. The frequently used decoupling capacitor is not appropriate because it has self-resonance characteristics due to the parasitic inductance. To realize resonance-free wideband decoupling, a high-attenuation power line (HAPL) is proposed. The HAPL has constant input impedance equal to its characteristic impedance, and has constant isolation unaffected by the phase constant and the terminal impedance. Furthermore, the HAPL contributes to the area reduction of the millimeter-wave circuits because it simultaneously acts as a power line and a decoupling device. The HAPL was fabricated with a 90 nm CMOS process. The proposed structure increases parallel conductance and capacitance using an MOS capacitor and its equivalent series resistance, therefore realizing high attenuation and resonance suppression while reducing characteristic impedance. With a 200-µm-long HAPL, Re (S11) was less than -0.9 and isolation was more than 25 dB, from 50 GHz to 90 GHz including unlicensed bands used for wireless personal area network and radar application. As a result, power-supply network with wideband decoupling is realized by simply connecting power-supply pads and feeding point via the HAPL. The HAPL is expected to contribute to the simple and compact design of millimeter-wave circuits.