Disclosed is a method for preventing the formation of a spiral mark in the inner side surface of a through-hole and achieving excellent appearance of the through-hole and high machining accuracy in the formation of the through-hole in a machining material by means for a drill-head equipped with a brazed cutting blade. A through-hole (H) is formed on a machining material (W) by means of a drill-head (1A) equipped with cutting blades (3A-3C) brazed onto blade mounting seats (13a-13c). The drill-head (1A) has a rounded portion (R) in an outer blade edge (31a) of the cutting blade (3A) for cutting the outermost peripheral area of the machining material (W).