A power supply device comprising a driving control device (100, 300) and an auxiliary control device that are installed in a vehicle, the driving control device including a steering control device and a turning control device, the vehicle including a main power source, an auxiliary power source, and a supply path (Lig), the auxiliary power source being a power source that stores electricity supplied from the main power source, the supply path being a path that supplies electricity from the main power source to electronic equipment inside the vehicle and being configured to be opened and closed according to a state of a start switch of the vehicle, wherein: the driving control device is a device that controls a state of equipment installed in the vehicle while using either the main power source or the auxiliary power source as a power source; and the auxiliary control device is a device that controls a state of the auxiliary power source; characterized in that the driving control device is configured to execute a storage process (S86, S88), a permission signal transmission process, and an initial value process, the storage process being a process of storing, in a storage device, a state of the start switch determined based on a signal from an outside of the driving control device, the permission signal transmission process being a process of transmitting a permission signal when the state of the start switch stored in the storage device is an off state, the initial value process being a process of, at start-up of the driving control device, setting an initial value of the state of the start switch stored in the storage device to a value indicating an on state; and the auxiliary control device is configured to execute a permission signal reception process and a stop process, the permission signal reception process being a process of receiving the permission signal, the stop process being a process of putting control of supply of electricity from the auxiliary power source to the driving control device into an off state when the permission signal is received. the driving control device including a steering control device and a turning control device, the vehicle including a main power source, an auxiliary power source, and a supply path (Lig), the auxiliary power source being a power source that stores electricity supplied from the main power source, the supply path being a path that supplies electricity from the main power source to electronic equipment inside the vehicle and being configured to be opened and closed according to a state of a start switch of the vehicle, wherein: the driving control device is a device that controls a state of equipment installed in the vehicle while using either the main power source or the auxiliary power source as a power source; and the auxiliary control device is a device that controls a state of the auxiliary power source; characterized in that the driving control device is configured to execute a storage process (S86, S88), a permission signal transmission process, and an initial value process, the storage process being a process of storing, in a storage device, a state of the start switch determined based on a signal from an outside of the driving control device, the permission signal transmission process being a process of transmitting a permission signal when the state of the start switch stored in the storage device is an off state, the initial value process being a process of, at start-up of the driving control device, setting an initial value of the state of the start switch stored in the storage device to a value indicating an on state; and the auxiliary control device is configured to execute a permission signal reception process and a stop process, the permission signal reception process being a process of receiving the permission signal, the stop process being a process of putting control of supply of electricity from the auxiliary power source to the driving control device into an off state when the permission signal is received. the driving control device is a device that controls a state of equipment installed in the vehicle while using either the main power source or the auxiliary power source as a power source; and the auxiliary control device is a device that controls a state of the auxiliary power source; characterized in that the driving control device is configured to execute a storage process (S86, S88), a permission signal transmission process, and an initial value process, the storage process being a process of storing, in a storage device, a state of the start switch determined based on a signal from an outside of the driving control device, the permission signal transmission process being a process of transmitting a permission signal when the state of the start switch stored in the storage device is an off state, the initial value process being a process of, at start-up of the driving control device, setting an initial value of the state of the start switch stored in the storage device to a value indicating an on state; and the auxiliary control device is configured to execute a permission signal reception process and a stop process, the permission signal reception process being a process of receiving the permission signal, the stop process being a process of putting control of supply of electricity from the auxiliary power source to the driving control device into an off state when the permission signal is received.