A self-power circuit for a protecting relay, characterized in that the self-power circuit comprising: a rectifying circuit section configured to rectify an alternating current from a current transformer that detects amount of an electric current flowing through an electric power line of an electric power system; a power source circuit section connected to an output terminal of the rectifying circuit section and configured to include a comparator comparing whether an output voltage from the rectifying circuit section exceeds a reference voltage and a semiconductor switch at a first end connected to a ground and configured to be switched by an output from the comparator, the power source circuit section is configured to supply the output voltage from the rectifying circuit section as a constant voltage to a microcomputer unit of the protecting relay; the self-power circuit is characterised in that it further comprises: an inductor connected to the semiconductor switch of the power source circuit section in series and between the output terminal of the rectifying circuit section and a second end of the semiconductor switch, wherein the inductor is configured to, when a direct current supplied from the rectifying circuit section is rapidly increased, gradually increase the direct current to flow to thus have an increased voltage thereacross; and a first bi-directional diode connected between an output terminal of the power source circuit section and a ground and turned on to form a bypass path of an electric current when a direct current voltage supplied from the power source circuit section to the microcomputer unit of the protecting relay is rapidly increased. a rectifying circuit section configured to rectify an alternating current from a current transformer that detects amount of an electric current flowing through an electric power line of an electric power system; a power source circuit section connected to an output terminal of the rectifying circuit section and configured to include a comparator comparing whether an output voltage from the rectifying circuit section exceeds a reference voltage and a semiconductor switch at a first end connected to a ground and configured to be switched by an output from the comparator, the power source circuit section is configured to supply the output voltage from the rectifying circuit section as a constant voltage to a microcomputer unit of the protecting relay; the self-power circuit is characterised in that it further comprises: an inductor connected to the semiconductor switch of the power source circuit section in series and between the output terminal of the rectifying circuit section and a second end of the semiconductor switch, wherein the inductor is configured to, when a direct current supplied from the rectifying circuit section is rapidly increased, gradually increase the direct current to flow to thus have an increased voltage thereacross; and a first bi-directional diode connected between an output terminal of the power source circuit section and a ground and turned on to form a bypass path of an electric current when a direct current voltage supplied from the power source circuit section to the microcomputer unit of the protecting relay is rapidly increased. an inductor connected to the semiconductor switch of the power source circuit section in series and between the output terminal of the rectifying circuit section and a second end of the semiconductor switch, wherein the inductor is configured to, when a direct current supplied from the rectifying circuit section is rapidly increased, gradually increase the direct current to flow to thus have an increased voltage thereacross; and a first bi-directional diode connected between an output terminal of the power source circuit section and a ground and turned on to form a bypass path of an electric current when a direct current voltage supplied from the power source circuit section to the microcomputer unit of the protecting relay is rapidly increased.