A vehicular electric power supply system that is mounted in a vehicle, comprising: a load is configured to be supplied with electric power from one or both of a first-series electric power supply line and a second-series electric power supply line that are provided independently of each other; a first battery is configured to be switchable by switches between a state connected only to the first power supply line and a state connected only to the second power supply line; a second battery is configured to be switchable by switches between a state connected only to the first power supply line and a state connected only to the second power supply line; and a connection control device is configured to control connection of the first battery and the second battery to the first-series electric power supply line and the second-series electric power supply line, based on a state of the vehicle, wherein the state of the vehicle includes a manual operation state where the vehicle runs through manual operation by a driver of the vehicle, characterized in that: the state of the vehicle further includes an automatic operation state where the vehicle runs through automatic operation; the connection control device is configured to connect both the first battery and the second battery in parallel to the first-series electric power supply line when the vehicle is in the manual operation state; and the connection control device is configured to, when the vehicle is in the automatic operation state, connect the first battery to the first-series electric power supply line and connect the second battery to the second-series electric power supply line. a load is configured to be supplied with electric power from one or both of a first-series electric power supply line and a second-series electric power supply line that are provided independently of each other; a first battery is configured to be switchable by switches between a state connected only to the first power supply line and a state connected only to the second power supply line; a second battery is configured to be switchable by switches between a state connected only to the first power supply line and a state connected only to the second power supply line; and a connection control device is configured to control connection of the first battery and the second battery to the first-series electric power supply line and the second-series electric power supply line, based on a state of the vehicle, wherein the state of the vehicle includes a manual operation state where the vehicle runs through manual operation by a driver of the vehicle, characterized in that: the state of the vehicle further includes an automatic operation state where the vehicle runs through automatic operation; the connection control device is configured to connect both the first battery and the second battery in parallel to the first-series electric power supply line when the vehicle is in the manual operation state; and the connection control device is configured to, when the vehicle is in the automatic operation state, connect the first battery to the first-series electric power supply line and connect the second battery to the second-series electric power supply line. the state of the vehicle further includes an automatic operation state where the vehicle runs through automatic operation; the connection control device is configured to connect both the first battery and the second battery in parallel to the first-series electric power supply line when the vehicle is in the manual operation state; and the connection control device is configured to, when the vehicle is in the automatic operation state, connect the first battery to the first-series electric power supply line and connect the second battery to the second-series electric power supply line. connect the first battery to the first-series electric power supply line and connect the second battery to the second-series electric power supply line.