A cylindrical piece of bentonite soaked in ample water, inbibition at both its ends being hindered, swells gradually and attains the maximum height. This holds on long enough and can practically be taken to be an ultimate linear swelling value. In other words, the abovementioned value can be applied to discriminate bentonite from other substances as well as to grade its own various kinds. For practical use the author proposes a new index called “Bentonite Equivalent”.