IN-VEHICLE SYSTEM AND JUNCTION BOX

An in-vehicle system is provided with a front J/B and a first voltage detection unit. The front J/B has a main relay unit, a second voltage detection unit, and a second MCU. Based on detection results detected by the first voltage detection unit and a second voltage detection unit when the main relay unit is subjected to on/off operations, the second MCU monitors whether each of the main relay unit, the first voltage detection unit, and the second voltage detection unit has abnormality or not.