PURPOSE: To remarkably reduce uneven blast by oscillating plural jet nozzles in the cross direction of a substrate to be machined at the constant speed in parallel to the substrate to be machined so as to keep the angles of the jet nozzles and the distances thereof from the substrate constant to be machined.

CONSTITUTION: When a motor 28 is driven, an eccentric cam part 23 is caused to make eccentric rotational motion through a belt 27, and the eccentric rotational motion is converted to the cross-directional rectilinear oscillating motion and transmitted to a supporting rod 14a and a nozzle plate 14 through a connecting plate 16 by an arm part 21. Abrasive material is radially jetted from the respective jet nozzles 13 to the back of a semiconductor wafer 11 transported on a transport path 12, with its back side facing upward, thereby blast machining is carried out for the purpose of the back gettering. In this case, the semiconductor wafer 11 is transported at a constant transport speed and the jet nozzles 13 are oscillated and moved in the cross-direction at a designated speed, so that the semiconductor wafer 13 is subjected to machining while drawing its locus of a substantially sine curve.