To provide an air compression device safe, causing little failures, easy in operation and maintenance that generates a high pressure air at a low cost without using a centrifugal force or a reciprocating action with a simple structure without inner equipment such as a blade, a gear, a piston, a screw or a valve by minimizing power to be used and using a high head of water flow and a depth of water.

This air compressing device by gas-liquid multi-phase flow allows a natural flow or a gas-liquid multi-phase flow from a low pressure pump fed inside a pipe to flow from a high place to a low place or forcibly feed the low pressure gas-liquid multi-phase flow from a water surface in to a deep part in water. The automatically highly-pressurized gas-liquid multi-phase flow by the high head or the depth of water is fed to a gas-liquid separation chamber for separating only a highly-pressurized gas to secure it in a high pressure gas vessel on the water surface or on the ground.