A current sensor 73 detects a current flowing through a first buss bar 11. When a current value detected by the current sensor 73 became equal to or greater than a threshold current value, a CPU 74 outputs a driving control signal to a driving circuit 77, and the driving circuit 77 operates an, ignitor 29 through a second substrate 65 and a terminal 50. Therefore, the ignitor 29 ignites, a second projection 41 is melted by heat of a heating agent 27, a compression spring 34 is expanded and a thermite case 26 jumps up. Thus, electrical connection between the thermite case 26 and the first and second buss bars 11, 19 is interrupted. Further, since an outer container 61 in which the current sensor 73, the CPU and the driving circuit 77 are accommodated is integrally assembled with the cap 14a and the resin case 14b in which the thermite case 26, the ignitor 29, the compression spring 34 and the second projection 41 are accommodated, non-operational state of the ignitor 29 due to disconnection of electric wire is not generated.