A power circuit for an integrated circuit chip having a plurality of operation frequency modes, the power circuit varying a resonance point defined by a parasitic resistance, inductance and capacitance existing in a power supplying line, in accordance with an operation frequency to thereby prevent the operation frequency from being in accord with a resonance frequency. For instance, when the operation frequency is relatively high, the power circuit lowers the resonance point, and when the operation frequency is relatively low, the power circuit raises the resonance point. The power circuit may further include an encoder receiving an operation frequency mode signal, and emitting an output signal indicative of an operation frequency. The power circuit provides a power ensuring stable operation of an external circuit in a plurality of operation frequency modes.