A screw comprising: a head including a top surface, a conical surface opposite to said top surface, a plurality of cutting units extending circumferentially of said top surface at an outer edge thereof, a plurality of accommodation portions recessedly provided in said conical surface; a shank extending axially from said head and including a drill section opposite to said head; and a plurality of thread convolutions spirally disposed on said shank; characterized in that said head includes a plurality of ridge portions each situated between every two adjacent accommodation portions, said plurality of ridge portions protruding from said conical surface, said plurality of accommodation portions and said plurality of ridge portions being situated between said plurality of cutting units and said shank and being arranged in an alternating manner; and in that each of said plurality of cutting units includes a first wall defining a first edge, a second wall defining a second edge connected to said first edge, and a third edge defined along a junction of said first wall and said second wall, with said second edge extending from one first wall of one cutting unit to another first wall of another cutting unit adjacent to said one first wall, and said plurality of cutting units thereby being connected one after another, each of said plurality of accommodation portions including a main groove recessedly formed in said conical surface and extending in the direction of said shank and an auxiliary groove formed around an outer periphery of said main groove, an area of said auxiliary groove being smaller than an area of said main groove so that said auxiliary groove is different from said main groove, and each of said plurality of ridge portions thereby being defined along a junction of each adjacent pair of auxiliary grooves of every two adjacent accommodation portions and situated between every two adjacent main grooves. a head including a top surface, a conical surface opposite to said top surface, a plurality of cutting units extending circumferentially of said top surface at an outer edge thereof, a plurality of accommodation portions recessedly provided in said conical surface; a shank extending axially from said head and including a drill section opposite to said head; and a plurality of thread convolutions spirally disposed on said shank; characterized in that said head includes a plurality of ridge portions each situated between every two adjacent accommodation portions, said plurality of ridge portions protruding from said conical surface, said plurality of accommodation portions and said plurality of ridge portions being situated between said plurality of cutting units and said shank and being arranged in an alternating manner; and in that each of said plurality of cutting units includes a first wall defining a first edge, a second wall defining a second edge connected to said first edge, and a third edge defined along a junction of said first wall and said second wall, with said second edge extending from one first wall of one cutting unit to another first wall of another cutting unit adjacent to said one first wall, and said plurality of cutting units thereby being connected one after another, each of said plurality of accommodation portions including a main groove recessedly formed in said conical surface and extending in the direction of said shank and an auxiliary groove formed around an outer periphery of said main groove, an area of said auxiliary groove being smaller than an area of said main groove so that said auxiliary groove is different from said main groove, and each of said plurality of ridge portions thereby being defined along a junction of each adjacent pair of auxiliary grooves of every two adjacent accommodation portions and situated between every two adjacent main grooves.