In a vehicular air bag system, a diagnostic apparatus for testing a squib line is provided which has: a first diagnostic resistor R1 one end of which is connected to a terminal D+ of a squib; a first diagnostic switching means Q1 for connecting the other end of the R1 to an ignition voltage VDC; a second diagnostic resistor R3 one end of which is connected to the terminal D+ of the squib; a second diagnostic switching means Q2 for connecting the other end of the R3 to ground; and a CPU. The CPU measures the potentials VD+ and VD- at the squib, by switching the first and second diagnostic switching means Q1 and Q2 on and off in an appropriate manner, and calculates the values of various prescribed diagnostic parameters on the basis of the value of VDC and the values of the VD+ and VD-. Then, the values of the diagnostic parameters are compared with various prescribed evaluation threshold values to determine the source of a failure.