An anchor, comprising: an anchor bolt having a shaft provided with a male screw thread; a spreading sleeve having spreading parts provided at a tip portion of a tubular body capable of being fitted on a tip portion of said shaft so as to receive the tip portion of the shaft therein, the spreading parts being formed by a longitudinally cut slot so as to spread outwardly; and a cone nut provided at its inner side a screw through hole to threadedly engage with said male screw thread formed on said shaft, the cone nut having an outer diameter gradually reduced in the axial direction of said shaft, the cone nut being attached to the tip portion of said shaft while a smaller-diameter edge part of the cone nut is fitted in a tip portion of said spreading sleeve, wherein said shaft of said anchor bolt is composed of a thick shaft part connected to said bolt head, and a thin shaft part provided to be closer to the tip portion of said shaft than said thick shaft part, coaxial with said thick shaft part, and smaller in diameter than said thick shaft part, a step is formed between said thick shaft part and said thin shaft part, the thin shaft part is provided with a male screw thread to threadedly engage with said cone nut, said spreading sleeve can be fitted on said thin shaft part of said shaft so as to receive said thin shaft part therein, and has an inner diameter that makes said spreading sleeve contact said step, characterized in that the anchor further comprises a bolt head provided at a base portion of the shaft, wherein said shaft of said anchor bolt is rotated in response to rotation of said bolt head of said anchor bolt to make said cone nut travel into said spreading sleeve contacting said step, thereby making said spreading parts spread outwardly; the thick shaft part is provided with a rupture portion reduced in diameter, the rupture portion being provided at a position spaced by a predetermined length from said step, the rupture portion rupturing if torque of a predetermined value or more acts on said bolt head; and in that said anchor bolt is provided at the tip end of said thin shaft part with a shaft center projection being a conically projecting part of a shaft center. an anchor bolt having a shaft provided with a male screw thread; a spreading sleeve having spreading parts provided at a tip portion of a tubular body capable of being fitted on a tip portion of said shaft so as to receive the tip portion of the shaft therein, the spreading parts being formed by a longitudinally cut slot so as to spread outwardly; and a cone nut provided at its inner side a screw through hole to threadedly engage with said male screw thread formed on said shaft, the cone nut having an outer diameter gradually reduced in the axial direction of said shaft, the cone nut being attached to the tip portion of said shaft while a smaller-diameter edge part of the cone nut is fitted in a tip portion of said spreading sleeve, wherein said shaft of said anchor bolt is composed of a thick shaft part connected to said bolt head, and a thin shaft part provided to be closer to the tip portion of said shaft than said thick shaft part, coaxial with said thick shaft part, and smaller in diameter than said thick shaft part, a step is formed between said thick shaft part and said thin shaft part, the thin shaft part is provided with a male screw thread to threadedly engage with said cone nut, said spreading sleeve can be fitted on said thin shaft part of said shaft so as to receive said thin shaft part therein, and has an inner diameter that makes said spreading sleeve contact said step, characterized in that the anchor further comprises a bolt head provided at a base portion of the shaft, wherein said shaft of said anchor bolt is rotated in response to rotation of said bolt head of said anchor bolt to make said cone nut travel into said spreading sleeve contacting said step, thereby making said spreading parts spread outwardly; the thick shaft part is provided with a rupture portion reduced in diameter, the rupture portion being provided at a position spaced by a predetermined length from said step, the rupture portion rupturing if torque of a predetermined value or more acts on said bolt head; and in that said anchor bolt is provided at the tip end of said thin shaft part with a shaft center projection being a conically projecting part of a shaft center.