PURPOSE: To improve the accuracy of machining both ends of a bar material by inserting a bar in a chuck member with both end portions of the bar material exposed, and machining both end portions of the bar material at the same time, with the bar material fixed and held near both end portions thereof by the chuck member.

CONSTITUTION: In case of machining a bar material (a), the bar material (a) is inserted from a hole portion 15 of the first chuck portion 7, and inserted in a chuck member 2, with both end portions of the bar material (a) to be machined exposed. Subsequently, pressurized oil is supplied to the respective oil sumps of the first and second moving members 5A, 5B through oil paths 45, 55 thereby to move movable plates 43, 52 in directions of A and B. The chuck member 2 is moved in direction of A by the movable plate 43 through a bearing 47 and a collar portion 16 of a coupling portion 6, so that a turncated cone portion 13 of the chuck portion 7 engaged with a conical opening portion 3 is reduced in diameter to retain and fix one end portion of the bar material (a). Similarly, a truncated cone portion 19 of the second chuck portion 8 is diameter by the movable plate 52 to hold and fix the other end portion of the bar material (a).