An electric circuit breaker device includes an igniter, a rod-like projectile, and a conductor portion for forming a part of an electric circuit, and an insulating closed space. The conductor portion includes a first and second connection portions and a cut portion at an intermediate portion, and is disposed with a surface of the cut portion. The rod-like projectile has an end portion facing the cut portion. The insulating closed space has an opening portion facing the conductor portion, a closed end surface opposite to the opening portion in the housing axial direction, and four side surfaces between the opening portion and the closed end surface. A width (W1) of the opening portion of the insulating closed space and a width (L1) of the end portion of the rod-like projectile have a relationship of W1>L1 and W1−L1≥0.25 mm.