The present invention is a tire air-injection device provided in the car for easily and rapidly injecting air into a tire to maintain the tire pressure at an optimal level when tire pressure drops below a reference value. A compressor provided in the car is connected to an engine of the car through a power transfer means by operating a main switch, which is provided at the outside of the car. Compressed air is injected in the tire by the compressor into the tire through a flexible hose of which one end is connected to the compressor. A relief valve, which is provided for regulating pressure of the air flowing through the hose.