A compressed gas supplier has a power-free decompression device and an expansion chamber. The power-free decompression device decompresses a gas in a high-pressure source into a decompressed gas. The expansion chamber connects to the power-free decompression device and receives and stores the decompressed gas. The pneumatic tool is driven by the decompressed gas in the expansion chamber. Thus, the compressed gas supplier for the pneumatic tool is small and easy to be carry. In addition, the decompressed gas stored in the expansion chamber is also benefit for supplying decompressed gas to the pneumatic tool that needs much gas to drive.