To provide a workpiece fixing device which is for use in machining a product workpiece that requires a constant machining dimension in a longitudinal direction of a long material, and can set a distance from a material edge to a machining starting point to a constant value.

The workpiece fixing device 1 is formed of a clamp mechanism 3 for clamping the workpiece W, and a mount 2 on which the clamp mechanism 3 is arranged. The workpiece fixing device comprises: a positioning plate 6 arranged on a lateral side of the clamp mechanism 3 and abutting on one edge of the workpiece W; an energizing mechanism 4 arranged on the mount 2, for energizing the other edge of the workpiece W to the positioning plate 6; a detecting means 5 for detecting a protruding dimension of the workpiece W pinched by the energizing mechanism 4 and the positioning plate 6, from the clamp mechanism 3 to the energizing mechanism 4; and a control means 8 for comparing the protruding dimension detected by the detecting means 5 with a preset reference dimension, and moving a machining tool 9 by a tool moving means 7 for moving the machining tool 9 by a difference between the protruding dimension and the reference dimension, in a work core direction.