In a method of producing a molding die for use in producing honeycomb structure bodies, a reference position is determined as a relative position on a workpiece between a pillar of a high-pressure water jet and a reference hole. The reference hole is penetratingly formed in a workpiece. The outer periphery of the pillar passes through the reference hole without contacting the inner wall and opening edge of the reference hole. A slit groove formation position is aligned with a pillar formation position by relatively shifting the pillar to the workpiece based on coordinate data where the reference position becomes its origin. Each slit groove is formed in the workpiece by supplying the pillar onto the slit groove formation position on the workpiece while irradiating and scanning a laser beam into the pillar of a high-pressure water jet many times along the slit groove formation position on the workpiece.