To provide a machining workpiece holding assembly easy in part-to- part adjustment and capable of disposing workpieces accurately in a working area inside a machining device.

A workpiece holding assembly for machining device comprises an adjustable rail clamp 14 mounted on a rail 12 fixed to the machining device. The rail clamp 14 further comprises a label connection means released rapidly from a leveler head 16. The leveler head 16 comprises a connection part which forms a connection corresponding to a leveler connection means. The leveler head 16 comprises an inclination adjusting means which tilts the leveler head 16 to a desired angular position and a horizontally adjusting means which slides the leveler head 16 horizontally relative to a workpiece clamp means 18. The leveler head 16 further comprises a vertically adjusting means which moves the leveler head 16 vertically relative to the rail clamp.