An electric circuit breaker device includes: a housing including an accommodating space inside; a projectile configured to be projected from a projectile accommodating portion toward an arc-extinguishing region portion side by energy received from the igniter; and a conductor piece provided at the housing and forming a portion of an electric circuit, the conductor piece including, at a portion of the conductor piece, a cutoff portion to be cut off by the projectile. The projectile includes, on a distal end side, a cutting-off surface configured to cut off the cutoff portion during actuation of the igniter. Of the conductor piece, at least one of a first surface facing a projectile accommodating portion side or a second surface facing the arc-extinguishing region portion side is formed with a cutting-off assisting groove at a predetermined expected cutting-off position where the cutoff portion is pressingly cut by the cutting-off surface during actuation of the igniter.