PURPOSE: To make it possible to fix an adapter into an effective image device smaller than the originl effective image size by inserting a lens system having a positive refractive index as a whole into the adapter for mount conversion, making the focal distance of an objective lens short, and reducing the F number.

CONSTITUTION: Adapter for mount conversion of this invention is used when, for instance, the convertible lens of 35mm camera is fixed into cine camera, and TV camera, etc. Lens system 7 which has a positive refractive index as a whole is inserted into the adapter 4 for mount conversion. Besides, 5 is the first mount which matches with objective lens 1, and 6 is the second mount suitble for optical device. 5 and 6 are different mounts. By this constitution, the adapter itself has relay multiplication. As this relay multiplication is less than 1,the focal distance of the objective lens is shortened, and at the same time, F number is reduced. In other words, the inherent image angle of the objective lens can be effectively used, hence a bright lens system can be obtained.