1. An imaging optical system which has at least one lens, at least one surface of the at least one lens being divided into at least one band region surrounding a periphery of an optical axis and a center region including the optical axis, wherein a boundary between the at least one band region and the center region is defined by a circle having the center located on the optical axis and a step is provided at the boundary such that a meridional image plane is brought close to a reference image plane of the imaging optical system.