An image pickup optical system includes in order from an object side, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power. The first lens has a biconvex shape. An image-side surface of the second lens has a shape such that, a concave surface is directed toward an image side. At least an image-side surface of the third lens has a shape such that, a convex surface is directed toward the image side. An aperture is disposed nearest to the object side, and the following conditional expression (1) is satisfied: 0.9