A method for manufacturing a lens having a refractive index distribution includes: a step of contacting a monomer with a structural member of a polymer, the monomer and a material obtained by polymerization of the monomer showing refractive indices which are different from the refractive index of the polymer; a step of diffusing the contacted monomer in the structural member; and a step of polymerizing the monomer. In the step of diffusing the contacted monomer in the structural member, the method further includes the steps of: irradiating light on at least 50% of one of the surfaces of the structural member which is parallel to the diffusion direction of the monomer, and measuring the intensity of the light passing through the structural member in a predetermined region, and when the measured intensity of the light reaches a predetermined value, the step of polymerizing the monomer is started.