PURPOSE: To improve a working accuracy, etc., by providing a mobile member on the guide base fitted on a machine base and by arranging a gage holder in the horizontal direction on the mobile member as well, further by fitting a butt positioning member to the gage holder.

CONSTITUTION: The back gage device 21 providing a pair of guide bases 93R, 93L is installed on the machine base of a work machine and the carriages 95R, 95L as for the moving member are arranged respectively on the bases 93R, 93L as well. Gage holders 105R, 105L are fixed freely position adjustably in the right and left direction to the carriages 95R, 95L. Butt positioning member 107R, 107L are fitted to each gage holder together with a sensor 107S. With this mechanism, the working accuracy of the work device is improved since the folding position of a work piece can correctly be located.