An electric circuit breaker that prevents damage due to increased internal pressure after an electric circuit is cut off and prevents an arc from leaking to the outside. Such a circuit breaker includes a housing, a cut portion that is arranged in the housing and forms a part of an electric circuit, a cutting member that cuts the cut portion, and a power source arranged on a first end portion side of the housing, the electric circuit breaker including a moving body including the cutting member, in which the housing includes a cylindrical portion capable of moving the moving body between the first end portion and a second end portion on a side opposite to the first end portion, in which the moving body is configured such that the cutting member provided in the moving body cuts a separation piece of the cut portion.