An onboard system that reduces a mental pressure on a driver includes a remaining amount detector that detects a remaining amount of a power source remaining in a vehicle, a lighting detector that detects that a low power source indicator provided in the vehicle lights up, a lighting remaining amount storage unit that stores, as a lighting remaining amount, the remaining power source amount detected by the remaining amount detector at a timing at which the lighting detector detects the lighting of the low power source indicator, a notification remaining amount detector that detects a remaining power source amount larger by a predetermined amount than the lighting remaining amount stored in the lighting remaining amount storage unit, and a notifier that notifies information for promoting supply of the power source after the remaining power source amount larger than the lighting remaining amount by the predetermined amount is detected before the lighting remaining amount is detected.