An image taking optical system includes, in order from the object side, a stop, a first lens having a biconvex shape and having a positive refractive power, a second lens having a concave surface facing the image side and having a negative refractive power, a third lens having a convex surface facing the image side and having a positive refractive power, and a fourth lens having a negative refractive power. The image taking optical system satisfies the conditional expression “0.6