The practice, data-analysis methods and the data-interpretation techniques of the “vacuum-permeability test”, which is a method of detecting the loosened zone in rock mass by evacuating air from a sealed section of a borehole drilled in the rock mass and obtaining the air permeability distribution characteristics, have been described and examined in detail. A practical and reliable approach is proposed, and is shown that it is a promising evaluation method of loosened zone by conducting fields tests at various sites and comparing the results of existing methods and already known data.