To provide an optical system which is not easily influenced by surface roughness even when an aspheric lens is used, can be easily manufactured and is not easily influenced by environmental changes such as a temperature change.

The optical system 10 is constituted of a 1st lens 20 and a glass-made 2nd lens 30 having positive refractive power which are successively arranged from the object side and the shapes of all optical faces of the 1st and 2nd lenses 20, 30 are rotationally symmetrical about the optical axis. The 1st lens 20 has a meniscus shape turning its convex surface to the image surface side, the optical surface of the object side has an aspherical shape and both the optical surfaces of the object side and image surface side of the 2nd lens 30 are spherical shapes. When it is defined that the half field angle of a beam made incident on the optical system is θ[rad.], the image height of a beam emitted from the optical system is Y[mm] and the focal distance of the optical system is f[mm], the following inequality (1) is satisfied. The inequality (1) is -0.10<(Y-f×θ)/Y<0.10.