PURPOSE: To provide a method and system for measuring the moving gradient of underwater efficiently and accurately at a low cost using a single observation hole.

CONSTITUTION: A water moving gradient measuring equipment 6 comprising a rod shaft 5 carrying a neutron moisture gauge 9, a water pressure gauge 8, and air packers 7 disposed at the upper and lower ends of the rod shaft is inserted into an observation hole bored into a water retaining layer. The air packers 7 are inflated to stop water flow in the observation hole and then tracer liquid 19 is injected in order to measure the direction and the speed (v) of underground water flow. Water pressure increased by injection of the tracer liquid 19 and a time to be elapsed before natural water pressure is recovered are also measured and a water permeability (k) is calculated and then a moving water gradient is determined as follows; i=v/k.