An electric circuit breaker device comprising: an igniter provided to a housing; a projectile disposed in a cylindrical space formed in the housing, the projectile being movable in the cylindrical space by energy received from the igniter; and a conductor piece that is provided to the housing, forms a portion of an electric circuit, includes a cutoff portion to be cut off by the projectile in a portion thereof, and is disposed with the cutoff portion crossing the cylindrical space; characterized in that the electric circuit breaker device further comprises: an arc-extinguishing region positioned within the cylindrical space, on a side opposite to the projectile prior to actuation of the igniter with the cutoff portion interposed between the arc-extinguishing region and the projectile, and configured to receive the cutoff portion cut off by the projectile; and a coolant material having a fibrous form and disposed in the arc-extinguishing region, so that at the moment when the cutoff portion of the conductor piece is cut off by the projectile the cutoff portion can be instantaneously buried in the coolant material in the arc-extinguishing region and quenched by the coolant material, wherein the coolant material is a coolant material that removes thermal energy of the arc generated when the projectile cut off the cutoff portion, and the coolant material is formed from a metal fiber material. an igniter provided to a housing; a projectile disposed in a cylindrical space formed in the housing, the projectile being movable in the cylindrical space by energy received from the igniter; and a conductor piece that is provided to the housing, forms a portion of an electric circuit, includes a cutoff portion to be cut off by the projectile in a portion thereof, and is disposed with the cutoff portion crossing the cylindrical space; characterized in that the electric circuit breaker device further comprises: an arc-extinguishing region positioned within the cylindrical space, on a side opposite to the projectile prior to actuation of the igniter with the cutoff portion interposed between the arc-extinguishing region and the projectile, and configured to receive the cutoff portion cut off by the projectile; and a coolant material having a fibrous form and disposed in the arc-extinguishing region, so that at the moment when the cutoff portion of the conductor piece is cut off by the projectile the cutoff portion can be instantaneously buried in the coolant material in the arc-extinguishing region and quenched by the coolant material, wherein the coolant material is a coolant material that removes thermal energy of the arc generated when the projectile cut off the cutoff portion, and the coolant material is formed from a metal fiber material. an arc-extinguishing region positioned within the cylindrical space, on a side opposite to the projectile prior to actuation of the igniter with the cutoff portion interposed between the arc-extinguishing region and the projectile, and configured to receive the cutoff portion cut off by the projectile; and a coolant material having a fibrous form and disposed in the arc-extinguishing region, so that at the moment when the cutoff portion of the conductor piece is cut off by the projectile the cutoff portion can be instantaneously buried in the coolant material in the arc-extinguishing region and quenched by the coolant material, wherein the coolant material is a coolant material that removes thermal energy of the arc generated when the projectile cut off the cutoff portion, and the coolant material is formed from a metal fiber material.