The present invention provides an electric circuit breaker device, wherein, in a cylindrical space penetrating from a first end to a second end of a housing made of a synthetic resin, an igniter, a projectile made of a synthetic resin, and a conductor piece forming part of an electric circuit are arranged axially in this order from the first end of the housing, and the device has an insulating space for receiving a cut portion of the conductor piece between the second end of the housing and the conductor piece, wherein the conductor piece is a plate piece composed of connection sections at both ends thereof and a cutting section as an intermediate portion, and is disposed so that the cutting section has a surface perpendicular to the axial direction of the housing; the projectile is disposed to face the surface of the cutting section of the conductor piece in the axial direction of the housing; and a cylinder made of a metal for reinforcing the housing is disposed between the projectile and an inner wall surface of the housing.