Diffraction optical element, manufacturing method thereof, and optical apparatus

An optical apparatus comprising a housing, and an optical system disposed inside the housing, wherein

• the optical system is configured to have a diffraction optical element and a lens, and
• the diffraction optical element is a diffraction optical element comprising a base material, a first resin layer and a second resin layer which are laminated in order, wherein
• the first resin layer is configured to have a diffraction grating shape,
• the diffraction grating shape is configured to form a plurality of concentric annular sections when planarly viewed from a lamination direction of the diffraction optical element,
• the second resin layer is configured to comprise a first portion and a second portion,
• the first portion is configured to be provided on at least a first annular section on the first resin layer,
• the first annular section is configured to be a circle surrounding a center of the diffraction optical element among the plurality of annular sections,
• the second portion is configured to be continuously provided from above the first portion to above a region including a periphery of the first resin layer, and
• a difference between a refractive index of the second portion on a center of the first annular section and a refractive index of the second portion on a circumference of the first annular section is within 0.0005.