To provide an Albada type finder optical system excellently compensating the curvature of field of an Albada optical system by making the surface, on the object side of a positive single lens having an Albada frame, an aspherical surface.

This optical system is composed, in order from the object side, of a first to a fourth lenses G1G4, an Albada reflection surface is provided on a surface r6 on time pupil E side of the third lens G3 and an Albada frame (visual field frame) FA and an AF frame FB are provided on the surface r7 on the object side of the fourth lens G4. The surface r7 on the object side of the fourth lens G4 is an aspherical surface being concave shape to the object side. A visual field mask is not closely stuck to the lens surface r7. The Albada frame and the lens surface r7 are hardly scratched at the time of forming the frame and the loss of the lens due to the scratch, etc.. is reduced. Consequently, a defective rate is lowered. massproductivity is improved and a low cost is attained. The deterioration of optical performance due to the scratch is prevented. It is not necessary to add a lens for compensating the curvature of field of the Albada optical system.