a hardness tester for measuring hardness of a material by pressing an indenter attached to the lower end of a load application shaft into the test specimen . a reference cylinder is coaxially and slidably fitted around the load application shaft , and a datum plane , to be in contact with the specimen , is formed at the lower end of the reference cylinder , so as to limit the penetration depth of the indenter into the specimen to a predetermined constant depth . a stop device is provided for regulating the descending of the load application shaft relative to the reference cylinder . a load detector for hardness measurement is interposed on the load application shaft , to measure the hardness of the specimen by measuring the force applied to produce an indentation of a predetermined size on it .