A tool holder using a shape memory alloy, comprising: a holder body including: a coupling portion at a rear side which is coupled to a chuck means; a clamping portion at a front side which has a bore into which a shank portion of a tool is inserted, a plurality of slits formed on an external circumferential surface of the clamping portion, communicating with the bore, and arranged in a circumferential direction; and a reinforcing portion in which an axial shape of an external circumferential surface between the coupling portion and the clamping portion has any one shape of a tapered shape, a two-dimensional curved line shape, and a horizontal line shape; and a shape memory alloy ring which tightens or loosens the clamping portion in a radial direction of the axis while being heated or cooled in a state in which the shape memory alloy ring is fitted on the slits of the clamping portion; characterised in that the clamping portion of the holder body has an external thread formed on a tip portion of the external circumferential surface and in that the tool holder further includes: a spacer which is interposed between the reinforcing portion of the clamping portion and the shape memory alloy ring; and a locking cap which is thread-coupled to the external thread portion of the clamping portion by an internal thread formed on an inner circumferential surface and prevents the shape memory alloy ring from being moved away. a holder body including: a coupling portion at a rear side which is coupled to a chuck means; a clamping portion at a front side which has a bore into which a shank portion of a tool is inserted, a plurality of slits formed on an external circumferential surface of the clamping portion, communicating with the bore, and arranged in a circumferential direction; and a reinforcing portion in which an axial shape of an external circumferential surface between the coupling portion and the clamping portion has any one shape of a tapered shape, a two-dimensional curved line shape, and a horizontal line shape; and a shape memory alloy ring which tightens or loosens the clamping portion in a radial direction of the axis while being heated or cooled in a state in which the shape memory alloy ring is fitted on the slits of the clamping portion; characterised in that the clamping portion of the holder body has an external thread formed on a tip portion of the external circumferential surface and in that the tool holder further includes: a spacer which is interposed between the reinforcing portion of the clamping portion and the shape memory alloy ring; and a locking cap which is thread-coupled to the external thread portion of the clamping portion by an internal thread formed on an inner circumferential surface and prevents the shape memory alloy ring from being moved away. a coupling portion at a rear side which is coupled to a chuck means; a clamping portion at a front side which has a bore into which a shank portion of a tool is inserted, a plurality of slits formed on an external circumferential surface of the clamping portion, communicating with the bore, and arranged in a circumferential direction; and a reinforcing portion in which an axial shape of an external circumferential surface between the coupling portion and the clamping portion has any one shape of a tapered shape, a two-dimensional curved line shape, and a horizontal line shape; and a shape memory alloy ring which tightens or loosens the clamping portion in a radial direction of the axis while being heated or cooled in a state in which the shape memory alloy ring is fitted on the slits of the clamping portion; characterised in that the clamping portion of the holder body has an external thread formed on a tip portion of the external circumferential surface and in that the tool holder further includes: a spacer which is interposed between the reinforcing portion of the clamping portion and the shape memory alloy ring; and a locking cap which is thread-coupled to the external thread portion of the clamping portion by an internal thread formed on an inner circumferential surface and prevents the shape memory alloy ring from being moved away.