An electronic circuit breaker comprising: a rectifier circuit which rectifies the secondary output of a current transformer that detects a current of a main circuit; and a constant voltage circuit which generates a constant voltage power source that is for driving a microcomputer and a tripping device of said circuit breaker by a current outputted from said rectifier circuit , said microcomputer including: an A/D conversion section to which a target measurement signal from said rectifier circuit is inputted; a threshold determination section which sets thresholds of an overcurrent, an instantaneous current, and a leakage current, the currents being to be flown to the main circuit, and determines whether or not the input signal to said A/D conversion section exceeds any of the thresholds; and a tripping signal output section which outputs a tripping signal to said tripping device when said threshold determination section determines that the input signal exceeds any of the thresholds of the overcurrent, the instantaneous current, and the leakage current, wherein said electronic circuit breaker comprises: a test signal output circuit which outputs a test signal that exceeds the set thresholds of said threshold determination section , the test signal being a sine wave signal that simulates the secondary output of said current transformer ; a first changeover switch which is connected in series between said current transformer and said rectifier circuit and whose opening/closing time is controlled by a control signal from said microcomputer , whereby a signal to be inputted to said rectifier circuit is periodically switched between a signal outputted from said current transformer and the test signal outputted from said test signal output circuit ; and a tripping output determination circuit which determines the operation state of said circuit breaker based on the output state of said tripping signal output section and the connection state of said first changeover switch . a rectifier circuit which rectifies the secondary output of a current transformer that detects a current of a main circuit; and a constant voltage circuit which generates a constant voltage power source that is for driving a microcomputer and a tripping device of said circuit breaker by a current outputted from said rectifier circuit , an A/D conversion section to which a target measurement signal from said rectifier circuit is inputted; a threshold determination section which sets thresholds of an overcurrent, an instantaneous current, and a leakage current, the currents being to be flown to the main circuit, and determines whether or not the input signal to said A/D conversion section exceeds any of the thresholds; and a tripping signal output section which outputs a tripping signal to said tripping device when said threshold determination section determines that the input signal exceeds any of the thresholds of the overcurrent, the instantaneous current, and the leakage current, a test signal output circuit which outputs a test signal that exceeds the set thresholds of said threshold determination section , the test signal being a sine wave signal that simulates the secondary output of said current transformer ; a first changeover switch which is connected in series between said current transformer and said rectifier circuit and whose opening/closing time is controlled by a control signal from said microcomputer , whereby a signal to be inputted to said rectifier circuit is periodically switched between a signal outputted from said current transformer and the test signal outputted from said test signal output circuit ; and a tripping output determination circuit which determines the operation state of said circuit breaker based on the output state of said tripping signal output section and the connection state of said first changeover switch . <EOS>