To enable molding of a hydrodynamic pressure producing part on the flat surface of a material by simple steps highly accurately and at low costs. A microdroplet 13 of an ultraviolet curable type ink from a nozzle head 11 by the ink jet method is caused to land or dropped on the upper end face 2b1 of the material 2b'. The nozzle head 11 and a light source 14 which irradiates a ultraviolet ray are disposed sequentially according to the direction of travel of the material 2b', and are provided at positions opposing the material 2b'. While the material 2b' is relatively slid, landing or dropping of the ink from the nozzle head 11 and curing of the ink by the ultraviolet ray irradiated from the light source 14 are gradually conducted according to in the direction of travel of the material 2b' to form the hydrodynamic pressure producing part.