The present invention is directed to an image-forming optical system. The image-forming optical system according to the present invention contains, in order from an object side: an iris diaphragm; a first lens having a positive refractive power and at least one aspheric surface; and a second lens having a negative refractive power and at least one aspheric surface. In accordance with the present invention, there is provided an image-forming optical system for cameras using an image pickup device that has a small number of lenses to easily realize the compactness and satisfies a requirement for a high optical performance.