The hobs are theoretically reconsidered on their peripheral taper and their rough cutting mechanism. The taper angle under a fixed length of hob varies according to the helix angle of gear and the setting angle and the cutting feed direction of hob. A method of determining the optimum taper shape is described on a calculation example. Additionally, in order to relieve the cutting load concentrated on the side edge of end tooth, a new type side taper hob, whose tooth thickness is gradually reduced in the rough cutting teeth, is proposed. Design example is given and satisfactory result is obtained in a hobbing with the trial hob.