An electric circuit breaker device includes in a housing, an igniter, a rod-like projectile, and a conductor portion configured to form a portion of an electric circuit disposed in a cylindrical space formed in the housing; and an insulating closed space formed between a second end portion of the housing and the conductor portion. The conductor portion is a plate portion having first and second connection portions at opposing ends and a cut portion in an intermediate portion between the first and second connection portions. A surface of the cut portion is orientated orthogonal to the housing axial direction; the rod-like projectile is disposed aligned with a surface of the cut portion in the housing axial direction; and a width (W1) of the insulating closed space and a width (W2) of an end portion of the rod-like projectile have relationships W1>W2 and W1−W2≤0.25 mm.