Provided are a wafer scale lens, which is so short in an optical total length with respect to an image height that it can correct an aberration satisfactory, and an optical system including the wafer scale lens and having a thin lens element on the side closest to the image. The optical system includes a first lens having a positive refractive power relative to an object, and a second lens arranged on the side of the image of the first lens and having a recessed shape on the side of the object. At least one lens is arranged on the side of the second lens. When that one of the lenses arranged on the side of the image of the second lens, which is arranged on the side closest to the image, is an i-th lens (i>3), this i-th lens includes an i-th lens flat plate and is formed on the object side of the i-th lens flat plate but has a refractive index different from that of the i-th lens flat plate, and a lens element having a convex shape on the object side where the main light ray of the maximum image height passes. Moreover, the ratio between the optical axial distance (or the lens distance) between a (i−1)-th lens and the i-th lens and the optical axis distance (or the optical total length) from the object side face of the first lens to the image face is 0.01 to 0.15.