PURPOSE: To obtain an optical system of which aberration is efficiently compensated and visibility can be continuously adjusted by shifting the 1st lens group on the optical axis of a finder in the finder optical system provided with the 1st lens group consisting of negative lenses and the 2nd lens group consisting of 2 positive lenses.

CONSTITUTION: In a finder optical system consisting of the 1st lens group A having the expansion block P of a penta roof prism and a negative lens G1 and the 2nd lens group B having 2 positive lenses G2, G3, a f2<f3 condition is satisfied and visibility is adjusted by shifting the 1st lens group on the optical axis of the finder when the focal distance of a positive lens on the penta roof prism in the 2nd lens group is f2 and the focal distance of the other positive lens is f3. In addition, it is perferable to satisfy R5<R6(R6<0) and R7≤R8 to compensate various averrations of an eyepiece when radiuses of curvature of respective lens surfaces in the 2nd lens group are R5, R6, R7, R8.