To provide a method for forming an intermediate formed body for manufacturing a light assist magnetic head in which a diffraction grading is highly precisely formed, the intermediate formed body, and the light assist magnetic head.

The molten resin passes through a center flow channel section M12, further passes through a runner forming section and a gate forming section, and reaches a cavity CV when sending the molten resin under pressure through the center flow channel section M12 from a supplying source N. In this case, the flow direction of the resin into an elongate cavity from the runner forming section becomes one direction and uniform flow can be secured compared to an annular cavity. Further as the straight groove M15 for transferring and forming the diffraction grating coincides with the flow direction of the resin in the cavity CV thereby by the synergy effect the resin flows into the bottom of the straight groove M15 and transferring property becomes superior.