PURPOSE: To obtain a pentagonal roof prism provided with an exact roof angle by providing a drop-out part forming a slight clearance between a die plate and a die piece on the rear of the ridgeline constituting a reflection surface and molding the prism.

CONSTITUTION: The exact roof angle in the ridgeline part 7 constituted of the reflection faces 3, 4 of a pentagonal roof prism 1 is obtained by plastic molding. In forming a cavity 8, die pieces 9, 10 are combined with a die plate 11, and when a high pressure is developed in the cavity 8 and a strong internal pressure acts on the pieces 9, 10 in the stage of injection molding, the pressure is received thoroughly in the greater part of the pieces 9, 10 by the plate 11 but in the part 7, a drop-out part 12 exists; therefore, said part deforms elastically and the packed resin assumes a sharp angle in the part 7. When the resin is cooled in this stage, the sinkage in the part 7 is large and after the cooling, the part 7 has an exact right angle and is free from droop.