After Aristotle, Greek science saw the rise of new scientific branch, applied-mechanics, which aimed at practical ends. Hero of Alexandria, who wrote many treatises on this field, and made a model of steam-engine in Pneumatika (Pneumatics), has been regarded as an outstanding figure in applied-mechanics. Meanwhile, being regarded as a mere mechanician, his scientific ability has been underestimated. So, Pneumatika which embraced all the contents of ancient pneumatics has been appreciated unjustly. The object of this article is to identify Alexandrian applied-mechanics and its background, and to re-appreciate Hero's scientific achievement by analyzing his Pneumatika. For the first step, the social and intellectual background will be considered. Then, the theories and practical contents of ancient pneumatics will be examined respectively, and its experimental character shall be revealed. In this respect, Hero might be estimated to have aimed at practical ends, for he made technical improvement in devicing machinery by experiment. But the examination of the ancient concept of "utility" and the cosmological background will show that he was not a mere mechanician. His deep interests in natural philosophy must not be neglected—he devoured theories, principles and many other metaphysical thesis. So Hero must be regarded as a traditional Greek scientist whose motive in science was the study of Nature itself.