A spindle head device (10) includes a spindle (12) and a tool retaining mechanism (16) detachably retaining a tool holder (14) on the spindle. The tool retaining mechanism includes a tool support member (52) having a receiving recess (50) detachably receiving the shank (46) of the tool holder, a tool clamp member (54) disposed to be displaceable with respect to the tool support member between an operative position and an inoperative position, and a mounting member (56) detachably mounting the tool support member and the tool clamp member to the spindle. The tool clamp member grips the shank of the tool holder and fixedly holds the shank to the receiving recess of the tool support member at an operative position, and, at an inoperative position, releases the shank to allow the tool holder to be attached or detached to or from the receiving recess.