To constitute an optical system of a small number of lenses so as to obtain the compact and inexpensive optical system by constituting the optical system as a four-group zoom optical system being negative, positive, negative and positive and using a radial type refractive index distribution lens in the optical system.

The 1st group has negative refractive power, is fixed at the time of varying power and acts to guide axial luminous flux and off-axis luminous flux to the 2nd group. The 2nd group has positive refractive power, is moved at the time of varying the power and mainly has power varying action. The 3rd group has the negative refractive power, is moved at the time of varying the power and mainly has action for correcting the positional deviation of an image surface caused by variable power. The 4th group has the positive refractive power, is fixed at the time of varying the power and has action for forming the luminous flux from the 3rd group into an image. A diaphragm is arranged on the object side of the 2nd group, and is moved integrally with the 2nd group in the case of variable power so as to restrain aberration fluctuation in the case of variable power. Therefore, the radial type refractive index distribution lens being the 2nd group and the diaphragm are moved integrally, so that the diameter of the radial type refractive index distribution lens is made small and cost is reduced.