We discussed a two lens optical system focused ion beam (FIB) employing a Ga-emitter (LMIS), whose acceleration voltage V-acc is ranged 30-100 kV, which will be used for milling and secondary ion microscope observation purposes. On such an optical system, we investigated the relation between minimum obtainable beam diameter and chromatic aberration coefficient against V-acc, and studied the possibility of improving FIB resolution by increasing the V-acc. The beam diameter is mainly determined by the size of a Gaussian image and the chromatic aberration, especially that of objective lens, if the beam current is very low. From this fact, we showed that the magnification optimization method which is one of the optical optimization methods can be greatly simplified at a lower beam current region. Using this simplified method, we summarized a guide line for evaluating an V-acc value from the standpoint of realizing a finer beam. Also given is information useful for designing a FIB column in consideration of optimized optical column design for realizing the; optimized magnification. Finally, calculating the attainable minimum beam diameter and optimum column length against V-acc values, we discussed the limitation of beam diameter by increasing the V-acc.