A parking brake system for electrically controlling a parking brake of a vehicle, the parking brake system comprising: a parking brake provided for each of the two wheels of at least one axle of the vehicle; an air dryer controller provided to an air dryer and configured to electrically control the parking brake, the air dryer being configured to dry compressed air for use in the parking brake; a pressure sensor configured to detect a pressure value of the dry compressed air that is supplied to the parking brake; and an actuator provided for each axle of the vehicle that is supplied with a parking brake and configured to actuate the parking brake with the dry compressed air in accordance with an electric signal from the air dryer controller, characterized in that the air dryer controller is configured to determine an operating condition of the parking brake based on the detected pressure value and to output an electric signal based on the determination to electrically control the parking brake, and wherein the actuator includes a solenoid valve configured to control supply of the compressed air to the parking brake, and a driver configured to actuate the parking brake by electrifying the solenoid valve based on the electric signal from the air dryer controller, the driver is electrically connected to the solenoid valve and actuates the parking brake by electrifying the solenoid valve, and the driver is configured to shift the solenoid valve to one of a filling position where dry compressed air is delivered to a downstream side, a neutral position where supply of the dry compressed air is blocked, and a discharge position where the dry compressed air on the downstream side is discharged to an outside by electrifying the solenoid valve. a parking brake provided for each of the two wheels of at least one axle of the vehicle; an air dryer controller provided to an air dryer and configured to electrically control the parking brake, the air dryer being configured to dry compressed air for use in the parking brake; a pressure sensor configured to detect a pressure value of the dry compressed air that is supplied to the parking brake; and an actuator provided for each axle of the vehicle that is supplied with a parking brake and configured to actuate the parking brake with the dry compressed air in accordance with an electric signal from the air dryer controller, characterized in that the air dryer controller is configured to determine an operating condition of the parking brake based on the detected pressure value and to output an electric signal based on the determination to electrically control the parking brake, and wherein the actuator includes a solenoid valve configured to control supply of the compressed air to the parking brake, and a driver configured to actuate the parking brake by electrifying the solenoid valve based on the electric signal from the air dryer controller, a solenoid valve configured to control supply of the compressed air to the parking brake, and a driver configured to actuate the parking brake by electrifying the solenoid valve based on the electric signal from the air dryer controller, the driver is electrically connected to the solenoid valve and actuates the parking brake by electrifying the solenoid valve, and the driver is configured to shift the solenoid valve to one of a filling position where dry compressed air is delivered to a downstream side, a neutral position where supply of the dry compressed air is blocked, and a discharge position where the dry compressed air on the downstream side is discharged to an outside by electrifying the solenoid valve.