PURPOSE:To find the water permeability of the ground, etc., automatically in a short time by raising and lowering the pressure of injected water while holding the maximum flow rate of injected water to a water permeability test hole, and finding the relation between the pressure and flow rate of the injected water continuously. CONSTITUTION:The water in a water tank 1 is sent to a pressure container 5 by a pump 2 and a water level controller 7 holds the water level. The pressure of compressed air from a compressor 13 is set to the lowest prssure by a pressure control valve 12. A two-way valve 11 is opened to flow the water into the water permeability test hole 10 through a flow rate detector 17 and a test is started. A maximum pressure gradient is found previously among pressure graddients with which the flow rate of the injected water is saturated, and the pressure gradient and maximum injected water pressure are set to a setter 21. A test controller 19 drives a prime mover 15 to operate the pressure control valve 12, and the pressure in the pressure container 5 is raised or lowered to the set pressure; and the current flow rate of the injected water is measured by the flow rate detector 17 to find the water permeability, which is recorded on a recorder 20.