A finder optical system includes an image taking lens configured to form a photogenic subject image, an erecting image forming member configured to convert the photogenic subject image an erecting image, and an ocular lens configured to enable observation the erecting image of the photogenic subject image therethrough, wherein the ocular lens includes, in an order from the erecting image forming member to the observation side, a first lens group of negative refracting power, a second lens group of positive refracting power and configured to move in an optical axis direction to perform diopter adjustment, and a third lens group having at least one positive lens and at least one negative lens and having a positive or negative overall refracting power, and wherein, when a focal length of the whole system as the ocular lens is at −1 diopter is denoted by f, a composite focal length of the negative lens or lenses of the third lens group is denoted by f3n, a conditional expression −0.55