A subminiature optical system used in a subminiature image sensor module is provided. The subminiature optical system has a first lens group, a second lens group and a third lens group sequentially arranged from the object side. The first lens group has a first lens of plus refractive power and a second lens of minus refractive power, the second lens being cemented to the first lens. The second lens group has a third lens where at least one refraction surface is aspherical and has minus refractive power. The third lens group has a fourth lens where at least one refraction surface is aspherical and has minus refractive power. The subminiature optical system can have a high resolution and a compact-sized lens construction with a small number of lens combinations. Also, the subminiature optical system can have a small dimension of an axial direction in a whole construction.