There are provided a heat-treating method capable of suppressing generation of slip in a CZ silicon single crystal wafer having a diameter of mainly 300 mm or more even under high temperature heat treatment to annihilate grown-in defects in the vicinity of a surface of the wafer, and an annealed wafer having a DZ layer in a surface layer of the wafer and oxide precipitates in the bulk thereof at a high density which exert a high gettering effect. First heat treatment of a silicon single crystal wafer manufactured from a silicon single crystal ingot pulled by means of a Czochralski method is performed at a temperature in the range of 600 to 1100.degree. C. to form oxide precipitates in the bulk of the wafer, and thereafter, second heat treatment is performed at a temperature in the range of 1150 to 1300.degree. C.