Upon reception of a work sensing command from a robot controller, an image processing unit obtains a two-dimensional image of thee whole work pallet including a work, with a CCD camera. The lock mechanisms of the socket and the header are respectively formed from the socket contacts and the header contacts, the lock mechanisms of the socket being formed by cutting away a portion of the socket contacts. Circularly shaped coils are provided to generate a time-constant, axially aligned magnetic field, Bz, in the chamber. A grating direction of each diffraction grating unit is substantially perpendicular to a main direction of light beams received by the diffraction grating unit. The annular base and subassembly are separately stored and connected just prior to delivery to the host annulus.