Abstract: A circuit breaker including a mechanism comprising an on-off mechanism unit for opening or closing the electric circuit and a trip mechanism unit for cutting off the closed circuit, the on-off and trip mechanism units being coupled independently of each other and relating to each other only when the circuit breaker is tripped, but not when open or closed, thereby aiming at diminution in the time from detection of abnormality to disconnection of contacts, an increase in a distance between the disconnected contacts, and stabilization of trip characteristic, thus improving the cutoff performance.