Extrusion screw tap

The purpose of the present invention is to suppress pressure on an engaging surface during tapping, and to suppress a reduction in durability and tool life. In the thread tap, a thread (18) of a cutting part is provided with an engaging surface (18a) which becomes an advancing side when being screwed into a pre-drilled hole of a workpiece (40) and a non-contact side surface (18b) which becomes a rear side, and in the cutting part, the press-in depth (Da) of the engaging surface (18a) of the thread (18) into the workpiece (40) is formed to be smaller than the press-in depth (Db) of the non-contact side surface (18b) of the thread (18). Therefore, during tapping, the reaction force (Fa) from the workpiece (40) applied to the engagement surface (18a) for plastically deforming the workpiece (40) is smaller than the reaction force (Fb) from the workpiece (40) applied to the non-contact side surface (18b), and thus the reaction force (Fb) applied to the engagement surface (18a) is suppressed, and the durability of