To make the overall length of an optical system short, decrease the number of lenses, make small variation in the size of an image when the image is defocused as a body shift in position, and make astigmatism and distortion small by using two positive lenses and arranging a stop more on an image side than a 2nd lens.

This optical system is composed of two elements in two groups, i.e., a 1st lens which is a positive biconvex lens, the 2nd lens which is a positive meniscus lens that is convex to the image plane side, and the stop which is provided on the image plane side of this 2nd lens. Then a condition of 0.24≤(d1+d2+d3+d4)/f≤7.20 is satisfied. Here, (f) is the focal length of the whole system, d1 the thickness of the 1st lens, d2 the air gap between the 1st and 2nd lenses, d3 the thickness of the 2nd lens, and d4 the air gap between the 2nd lens and stop. An entrance pupil position can be made sufficiently far away from the image side when the condition is satisfied, so when the body shift in position and the image is defocused, the variation in the size of the image can be reduced.