To provide a binding tool with which at least a given holding force can be secured for the bound sheets even as the energy required for operation is kept low. A binding tool is capable of binding multiple sheets that have been stacked between a pair of teeth by pressing a portion of said sheets with the inclined surfaces of said teeth while stretching the sheet material. The binding tool is characterized in satisfying at least one of the following: (1) the absolute value of the angle of inclination of the inclined surface with respect to the direction in which the teeth advance and retract is less than 38 DEG; (2) when the teeth are brought close to each other so that the length of a normal line that corresponds to the shortest distance between the inclined surfaces of the pair of teeth 0.09 mm, the length in the direction orthogonal to said normal line of the main region of pressurization by the two inclined surfaces is less than 0.18 mm; and (3) the sheet material stretching rate is greater than 1.32 and equal to or less than 1.60.