A vehicular power source controller, comprising: a semiconductor switching element; a control unit configured to: turn on and off the semiconductor switching element based on a first switching signal inputted in association with an operation of a first switch configured to operate a load of a vehicle and thereby switching on and off the power supplied from a power source of the vehicle to the load; and turn on and off the semiconductor switching element independently of the first switching signal when an abnormal current flowing through the load is detected and thereby the power supplied to the load has to be suppressed and thereby suppressing the power supplied to the load; characterized in that it further comprises a backup control circuit configured to turn on and off the semiconductor switching element when the control unit has an abnormality in operation, where the control unit cannot turn on and off the semiconductor switching element, synchronization with a second switching signal which is inputted in association with an operation of any other switch, other than the one associated with the load and thereby switching on and off the power supplied to the load. a semiconductor switching element; a control unit configured to: turn on and off the semiconductor switching element based on a first switching signal inputted in association with an operation of a first switch configured to operate a load of a vehicle and thereby switching on and off the power supplied from a power source of the vehicle to the load; and turn on and off the semiconductor switching element independently of the first switching signal when an abnormal current flowing through the load is detected and thereby the power supplied to the load has to be suppressed and thereby suppressing the power supplied to the load; characterized in that it further comprises a backup control circuit configured to turn on and off the semiconductor switching element when the control unit has an abnormality in operation, where the control unit cannot turn on and off the semiconductor switching element, synchronization with a second switching signal which is inputted in association with an operation of any other switch, other than the one associated with the load and thereby switching on and off the power supplied to the load. turn on and off the semiconductor switching element based on a first switching signal inputted in association with an operation of a first switch configured to operate a load of a vehicle and thereby switching on and off the power supplied from a power source of the vehicle to the load; and turn on and off the semiconductor switching element independently of the first switching signal when an abnormal current flowing through the load is detected and thereby the power supplied to the load has to be suppressed and thereby suppressing the power supplied to the load; characterized in that it further comprises a backup control circuit configured to turn on and off the semiconductor switching element when the control unit has an abnormality in operation, where the control unit cannot turn on and off the semiconductor switching element, synchronization with a second switching signal which is inputted in association with an operation of any other switch, other than the one associated with the load and thereby switching on and off the power supplied to the load.