Provided is a vehicle power source control device and system that supplies power from an auxiliary power source to an in-vehicle load while suppressing the consumption of power when a failure occurs in the main power source when a vehicle has not been started. When a drive signal generation unit generates an off-signal and a main power source has failed, a power source drive circuit in an in-vehicle power source control device controls a power source circuit such that power is supplied from an auxiliary power source to a control unit. If the drive signal generation unit is generating an off-signal and the main power source is in a failed state, the control unit uses power supplied from the auxiliary power source to control the relay to switch to the first stopped state, and controls the converter to switch to the second permissive state.