A tool holder includes a holder body having a trunk and a grip tube, a pressure ring fitted around the grip tube, a nut for axially moving the pressure ring, and a sleeve defining a pressure chamber around the outer periphery thereof. The trunk is formed with cylinders in which pistons are inserted respectively. The cylinders communicate with the pressure chamber through a communication channel. Hydraulic fluid is sealed in the cylinders, the communication channel and the pressure chamber. When the nut is rotated, the pressure ring is axially moved, thus radially compressing the grip tube. Simultaneously, the pistons are pushed into the respective cylinders, thus increasing the pressure in the pressure chamber. As a result, both a first holding hole formed in the grip and a second holding hole formed in the sleeve decrease in diameter, so that a tool inserted in these holes are held in position.