Some dense metal membranes are permeable only to hydrogen, useful to produce and purify hydrogen. Conventionally, hydrogen permeation flux through metal membranes is described as the square-root law. The permeability defined in the law is commonly used as a measure of membrane material. However, deviation from the law has been widely reported. We have extended the definition of permeability for precise description. This study applied it to a thick palladium membrane down to 0.01 MPa in absolute pressure. Experimental results showed that hydrogen permeation flux through the palladium membrane 200 μm thick did not obey the square-root law completely. From the permeation behavior, pressure-dependent permeability was evaluated. The resultant permeability was found to decrease and become a constant value, or intrinsic permeability, as pressure approached vacuum.