A spindle device of a machine tool having: a cylindrical spindle supported by a housing so as to rotate; a draw bar provided along the axis of rotation of the spindle to draw a tool holder to the distal end of the spindle to clamp it; and an unclamping device to move the draw bar in a direction of a rotational axis of the draw bar by utilizing a fluid pressure so as to unclamp the tool holder, the unclamping device comprises: a cylinder supported by the housing and provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar; a piston engaging with the cylinder, and reciprocally moving in an axial direction of the draw bar; and a pressure chamber (77, 78) being supplied with a predetermined-pressure fluid, and formed as the piston and the cylinder are engaged with each other, wherein the pressure chamber (77, 78) is provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar, characterized in that said piston of said unclamping device is rotatably supported in the inner circumferential surface of said spindle by a first radial bearing, said first radial bearing having an inner wheel and an outer wheel and being so constituted that the inner wheel or the outer wheel has no track groove enabling said piston to move in the axial direction of said spindle. a cylindrical spindle supported by a housing so as to rotate; a draw bar provided along the axis of rotation of the spindle to draw a tool holder to the distal end of the spindle to clamp it; and an unclamping device to move the draw bar in a direction of a rotational axis of the draw bar by utilizing a fluid pressure so as to unclamp the tool holder, the unclamping device comprises: a cylinder supported by the housing and provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar; a piston engaging with the cylinder, and reciprocally moving in an axial direction of the draw bar; and a pressure chamber (77, 78) being supplied with a predetermined-pressure fluid, and formed as the piston and the cylinder are engaged with each other, wherein the pressure chamber (77, 78) is provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar, characterized in that said piston of said unclamping device is rotatably supported in the inner circumferential surface of said spindle by a first radial bearing, said first radial bearing having an inner wheel and an outer wheel and being so constituted that the inner wheel or the outer wheel has no track groove enabling said piston to move in the axial direction of said spindle. a cylinder supported by the housing and provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar; a piston engaging with the cylinder, and reciprocally moving in an axial direction of the draw bar; and a pressure chamber (77, 78) being supplied with a predetermined-pressure fluid, and formed as the piston and the cylinder are engaged with each other, wherein the pressure chamber (77, 78) is provided in an annular space between the inner circumference of the spindle and the outer circumference of the draw bar, characterized in that said piston of said unclamping device is rotatably supported in the inner circumferential surface of said spindle by a first radial bearing, said first radial bearing having an inner wheel and an outer wheel and being so constituted that the inner wheel or the outer wheel has no track groove enabling said piston to move in the axial direction of said spindle.