Electric circuit breaker device

An electric circuit breaker device comprising:

• a housing serving as an outer shell member, the housing configured to enclose an accommodation space extending in one direction;
• an igniter provided in the housing;
• a projectile disposed in the housing, the projectile being configured to be projected from one end side of the accommodation space by energy received from the igniter, the projectile being configured to move along an extending direction of the accommodation space;
• and
• a conductor piece held by the housing, the conductor piece forming a part of an electric circuit, the conductor piece including a cutoff portion between a first connection end portion as one end portion and a second connection end portion as the other end portion, the cutoff portion being configured to be cut off by movement of the projectile, the cutoff portion being disposed across the accommodation space, wherein
• the projectile includes an engaging portion configured to engage with an engaged portion provided inside the housing when the projectile moves along the extending direction of the accommodation space.