A tool spindle structure comprising: a housing (12, 12a, 12b); a tool spindle adapted to be rotatably supported by a bearing (15, 115) in the housing; a clamp rod inserted in a through hole of the tool spindle and adapted to advance and retract along an axial direction of the tool spindle; a biasing member adapted to apply a biasing force for retracting the clamp rod to the tool spindle; and unclamping means (17, 18) adapted to apply a thrust force for advancing the clamp rod against the biasing force to the clamp rod, wherein: the clamp rod is retracted to fix a tool to the tool spindle, the tool being mounted at a distal end portion of the tool spindle, and the clamp rod is advanced to unfix the tool from the tool spindle, the tool spindle is supported in the housing such that displacement thereof is allowed along the axial direction; and a stopper member is adapted to restrict a displacement amount of the tool spindle that is displaceable with an axial load applied to the bearing by the advance of the clamp rod, wherein the stopper member is fixed to the housing, and a predetermined clearance (δ) is defined between the stopper member and the tool spindle characterised in that: the tool spindle includes a cylindrical portion at the distal end portion of the tool spindle, the cylindrical portion having a mount hole where the tool is to be mounted, and the clearance (δ) is defined between the stopper member and an endmost surface of the cylindrical portion. a housing (12, 12a, 12b); a tool spindle adapted to be rotatably supported by a bearing (15, 115) in the housing; a clamp rod inserted in a through hole of the tool spindle and adapted to advance and retract along an axial direction of the tool spindle; a biasing member adapted to apply a biasing force for retracting the clamp rod to the tool spindle; and unclamping means (17, 18) adapted to apply a thrust force for advancing the clamp rod against the biasing force to the clamp rod, the clamp rod is retracted to fix a tool to the tool spindle, the tool being mounted at a distal end portion of the tool spindle, and the clamp rod is advanced to unfix the tool from the tool spindle, the tool spindle is supported in the housing such that displacement thereof is allowed along the axial direction; and a stopper member is adapted to restrict a displacement amount of the tool spindle that is displaceable with an axial load applied to the bearing by the advance of the clamp rod, wherein the stopper member is fixed to the housing, and a predetermined clearance (δ) is defined between the stopper member and the tool spindle characterised in that: the tool spindle includes a cylindrical portion at the distal end portion of the tool spindle, the cylindrical portion having a mount hole where the tool is to be mounted, and the clearance (δ) is defined between the stopper member and an endmost surface of the cylindrical portion. the tool spindle includes a cylindrical portion at the distal end portion of the tool spindle, the cylindrical portion having a mount hole where the tool is to be mounted, and the clearance (δ) is defined between the stopper member and an endmost surface of the cylindrical portion.