An electric circuit breaker for interrupting an electric circuit of a vehicle includes a circuit break section having an explosive used for exploding a part of an electric circuit by explosion, a detonating device for exploding the explosive by application of electric power to the explosive, a collision detection device for detecting a collision of the vehicle and, a controller which explodes the explosive by supply of an electric current for detonating purposes to the detonating device in response to a detection signal from the collision detection device. The circuit breaker further includes a housing having a cylindrical member, such as an inner cylindrical portion, and a cover for covering the opening of the inner cylindrical portion. Connector terminals are formed on both ends of the conductor and a horizontal target section to be broken is formed in the middle of the conductor. The horizontal target area is disposed in front of the opening of the cylindrical member and the connector terminals are led to the outside of the housing. The explosive and a filament serving as means for heating the explosive are provided in the cylindrical member. The blasting force of the explosive that causes explosion by heating acts on the horizontal target area in a concentrated manner.