A clamp pallet (2) as a reference member is fixed to a table (1) of a machining center. A tapered positioning hole (12) is opened in a lower surface of a work pallet (3) as a movable member. An annular central pillar (21) is projected upward from the clamp pallet (2), and an intermediate member (22) is arranged between the central pillar (21) and the positioning hole (12). The intermediate member (22) is composed of a shuttle member (23), which is diametrically expandable and contractible and supported on the central pillar (21) vertically movably, and a tapered cylinder (26), which is attached to an outer periphery of the shuttle member (23) and makes a tapering engagement with the positioning hole (12). The shuttle member (23) is urged upward by means of an elastic member (24).