To provide a tool holder for gripping a tool by means of the pressure of a fluid.

A tool gripping mechanism 120 includes: a long cylindrical gripping sleeve 130 attached to a holder body 110; a tool gripping space 131 where the tool 200 is gripped; a slide ring 140 provided so as to be slidable in a long axis direction of the gripping sleeve; an elastic member 143 for elastically urging the slide ring toward the rear end of the holder; a fluid pressure chamber 150 where fluid pressure for urging toward a tip of the holder along the long axis direction of the gripping sleeve is generated; an engaged part 134 for tightening the tool in a direction intersecting with a long axis direction of the tool inserted into the tool gripping space 131; and a slope 142a of an engaging part 142 and a slope 134a of the engaged part 134 for converting the slide of the slide ring toward the rear end of the holder into a tightening operation, and also converting the slide of the slide ring in the fluid pressure chamber toward the tip of the holder into an operation of canceling the tightening.