An on-board storage system comprising: a storage device that has a controller, a NAND flash memory, and an interface; an electronic control unit that electronically controls a vehicle; and a sensor, wherein the electronic control unit communicates with the storage device through the interface, wherein the sensor transmits a detection result of the sensor to the electronic control unit, wherein the electronic control unit has a first interface for a main bus and a second interface for a sideband bus, wherein the interface of the storage device includes a third interface for the main bus and a fourth interface for the sideband bus, wherein the first interface for the main bus of the electronic control unit and the third interface for the main bus of the storage device are connected through the main bus which is configured with a PCI Express bus, wherein the second interface for the sideband bus of the electronic control unit and the fourth interface for the sideband bus of the storage device are connected through the sideband bus which is configured with a SD bus, wherein the main bus is connected to a connection module, wherein the electronic control unit transmits a command to start initialization of the NAND flash memory to the controller when a transmitted detection result of the sensor indicates a driving-start preliminary operation, wherein, when the electronic control unit transmits a command to start initialization of the NAND flash memory to the controller, the electronic control unit initializes the second interface for the sideband bus of the electronic control unit, the sideband bus, and the fourth interface for the sideband bus of the storage device, and wherein, after the initialization of the second interface for the sideband bus of the electronic control unit, the sideband bus, and the fourth interface for the sideband bus of the storage device, the electronic control unit transmits the command to start initialization of the NAND flash memory to the controller through the sideband bus. a storage device that has a controller, a NAND flash memory, and an interface; an electronic control unit that electronically controls a vehicle; and a sensor, wherein the electronic control unit communicates with the storage device through the interface, wherein the sensor transmits a detection result of the sensor to the electronic control unit, wherein the electronic control unit has a first interface for a main bus and a second interface for a sideband bus, wherein the interface of the storage device includes a third interface for the main bus and a fourth interface for the sideband bus, wherein the first interface for the main bus of the electronic control unit and the third interface for the main bus of the storage device are connected through the main bus which is configured with a PCI Express bus, wherein the second interface for the sideband bus of the electronic control unit and the fourth interface for the sideband bus of the storage device are connected through the sideband bus which is configured with a SD bus, wherein the main bus is connected to a connection module, wherein the electronic control unit transmits a command to start initialization of the NAND flash memory to the controller when a transmitted detection result of the sensor indicates a driving-start preliminary operation, wherein, when the electronic control unit transmits a command to start initialization of the NAND flash memory to the controller, the electronic control unit initializes the second interface for the sideband bus of the electronic control unit, the sideband bus, and the fourth interface for the sideband bus of the storage device, and wherein, after the initialization of the second interface for the sideband bus of the electronic control unit, the sideband bus, and the fourth interface for the sideband bus of the storage device, the electronic control unit transmits the command to start initialization of the NAND flash memory to the controller through the sideband bus.