Abstract: An optical system for a binocular is provided with an objective optical system, an erecting system and an eyepiece. The objective optical system includes a first lens group having positive refractive power, a second lens group having negative refractive power and a third lens group having positive refractive power. The second lens group and/or the third lens group is capable of moving in a direction perpendicular to an optical axis to stabilize an image. The objective optical system satisfies the following conditions: (1) 0.1<d12/f0 (2) 0.7<f0/f1<3.0 where, d12 is a distance between the first lens group and the second lens group, f0 is the focal length of the objective optical system, and f1 is the focal length of the first lens group.