To prevent the defective sand inclusion caused by an oblong hole spure by locating a green sand mold with a spure forming surface downward, and reciprocating a rotary drill in the longitudinal direction of the oblong hole spure.

A rotary drill 31 is started to rotate by a rotating means, and the rotary drill 31 is brought close to a mold 32 by a rotary drill elevating/ lowering means. The rotary drill 31 is controlled by a control means of an electric cylinder following the preliminarily inputted program, and reciprocated while cutting the mold 32 at a specified speed. When the rotary drill 31 is reciprocated, the rigidity of the device in the direction at right angle with respect to the advancing direction of the rotary drill 31 is increased by a linear guide, and a cut surface of a long hole gate 33 is not roughened. By reciprocating the rotary drill to form the oblong hole gate, a side surface of the gate is clearly cut, and no sand inclusion is present, improving the working effect.