To provide a manufacturing method of the transparent cap of an endoscope with a jacket sheath wherein both of the front and back surfaces of a part facing the observation window of the transparent cap to be fitted to the distal end of the jacket sheath can easily be formed in the state of a mirror surface and the transparent cap can easily be plastic-molded as one component.

As the parent mold of a part containing the part facing the observation window 2 of the transparent cap 12, the parent mold 12A' of a part in the state of the mirror surface is formed. As the parent mold of the other part of the transparent cap 12, the parent mold 12B' of the residual part is formed. The parent mold 12A' and the parent mold 12B' of the residual part are associated to form one transparent mold 12' of the transparent cap. In vacuum, liquid-state transparent resin is injected to the inner space of a transparent cap female mold 22 prepared by filling the surrounding of the parent mold 12' with liquid-state silicone rubber, and the resin is cured.