A grating forming method for forming a fine grating with a grating period of at most 1 .mu.m on a surface of a blank material including (1) feeding realatively a diamond cutting tool and the blank material in a groove extended direction while rotating the diamond cutting tool about the axial line and (2) cutting the surface of the blank material with a blade tip part of the diamond cutting tool to form the grooves while rotating and feeding the diamond cutting tool. When the fine grating with the grating period of "P" .mu.m is formed and the feed amount per revolution of the diamond cutting tool is set to be "S" .mu.m, the feed amount per revolution "S", with which the diamond cutting tool is fed in the groove extended direction while rotating the diamond cutting tool about the axial line, is set to satisfy the following conditional expression: S<2.9P.sup.3. This conditional are expression is applied to a blank material, a molding die, a molded product, and a master member for manufacturing a molding die.